Preparation method of alkali metal ion modified titanium silicalite zeolite for gas phase epoxidation of propylene and hydrogen peroxide

ABSTRACT

An alkali metal ion modified titanium silicalite zeolite for gas phase epoxidation of propylene and hydrogen peroxide and a preparation method thereof. The method includes, at first step: preparing an alkali metal hydroxide modification solution; at second step: conducting controlled hydrothermal treatment on a TS-1 zeolite matrix by using an alkali metal hydroxide solution; and at third step: conducting post-treatment on the hydrothermally modified TS-1 zeolite, including solid-liquid separation, washing, drying and calcining. In the washing process, the modified TS-1 zeolite wet material is washed with a low concentration alkali metal hydroxide solution; alkali metal ions are reserved on the silicon hydroxyl of the modified titanium silicalite zeolite; and an infrared characteristic absorption band of a framework titanium active center modified by the alkali metal ions is in a range above 960 cm−1 and below 980 cm−1.

TECHNICAL FIELD

The present invention belongs to the technical field of petrochemicalindustry, and relates to an alkali metal ion modified titaniumsilicalite zeolite for gas phase epoxidation of propylene and hydrogenperoxide and a preparation method thereof.

BACKGROUND

A titanium silicalite zeolite is a kind of silicate zeolite containingtitanium heteroatoms in its crystal framework. TS-1 is a very importantmember of a titanium silicalite zeolite family. MarcoTaramasso et al.first reported the synthesis method of TS-1 (GB2071071A, USP4410501,1983). It has an MFI topological structure, and has a ten-membered ringcross-channel system like the well-known silica-alumina zeolite ZSM-5.

A large number of basic researches show that the titanium heteroatomsexist in the TS-1 framework in isolated tetra-coordinate form. Thetitanium heteroatoms generate characteristic absorption of electronictransition from oxygen ligand to titanium site atom near 210 nm in theUV-visible diffuse reflectance spectroscopy and generate characteristicresonance absorption near 1120 cm⁻¹ in the UV Raman spectroscopy. Inaddition, framework titanium also generates characteristic absorption ofSi—O—Ti antisymmetric stretching vibration (or Si—O bond stretchingvibration disturbed by framework titanium) near 960 cm⁻¹ in theintermediate infrared region of the infrared spectrum. The localenvironment of the framework titanium can be changed. According to thereport of J. Catal., 1995,151, 77-86, after sodium exchange is conductedon TS-1 with 1 M NaOH solution at 25° C., the infrared characteristicpeak of the framework titanium of TS-1 zeolite near 960 cm⁻¹ disappears,and at the same time, new infrared characteristic absorption appears at985 cm⁻¹. Some literature suggests that the sodium exchange that occursin the strong alkaline solution is essentially the reaction of sodiumhydroxide and the silicon hydroxyl near the framework titanium(NaOH+Si—OH=Si—O⁻Na⁺+H₂O), which changes the local environment of theframework titanium, thereby influencing the infrared spectrumcharacteristics of the framework titanium.

A large number of application researches show that the TS-1 zeoliteobtained by introducing the titanium heteroatoms into the MFI frameworkhas unique catalytic oxidation performance and many uses. In short, TS-1can catalyze the epoxidation reaction of low-concentration hydrogenperoxide and a series of olefins to form epoxides. In addition, TS-1 canalso be used as a catalyst for hydroxylation of phenol, ammoxidation ofcyclohexanone and oxidation of alcohols to produce aldehydes andketones, and oxidation of alkane to produce alcohols and ketones. Neriet al. first reported a liquid phase epoxidation method of propylene bytaking methanol as a solvent and 30 wt. % hydrogen peroxide as anoxidant (U.S. Pat. No. 4,833,260, 1989), and obtained the result thatthe hydrogen peroxide conversion rate and the propylene oxide (PO)selectivity are larger than 90%. Clerici et al. systematically studiedthe reactions of various lower olefins and hydrogen peroxides catalyzedby TS-1 (J Catal, 1993, 140(1): 71), and pointed out the order of theliquid phase epoxidation rate of olefins in various solvents is:methanol>ethanol>tert-butanol. In 2008, the combination of the propyleneliquid phase epoxidation (HPPO) technology based on TS-1 titaniumsilicalite zeolite and methanol solvent with the anthraquinone methodfor producing hydrogen peroxide to build a green factory for producingpropylene oxide was first commercialized by Degussa and Uhde, and BASFand Dow, respectively (Ind. Eng. Chem. Res. 2008, 47, 2086-2090).

Although the TS-1 titanium silicalite zeolites synthesized by differentmethods can have the catalytic performance of olefin epoxidation, almostall of the practical TS-1 catalysts for the propylene liquid phaseepoxidation (HPPO) technology are prepared by using a nano TS-1 zeolitewith agglomerate particle size of 200-300 nm as a matrix. This kind ofzeolite is generally synthesized with high purity tetrapropylammoniumhydroxide (TPAOH) templating agent and according to the hydrothermalsynthesis method (classical method) introduced by Taramasso et al. (U.S.Pat. No. 4,410,501. 1983) and Thangaraj et al. (J Chem Soc Chem Commun,1992: 123). In order to achieve a better reaction performance,literature also has developed the so-called secondary hydrothermaltreatment technology for the nano TS-1 matrix by using an aqueoussolution containing a low concentration TPAOH templating agent.

The nano TS-1 zeolite synthesized by the classical method in thepropylene liquid phase epoxidation (HPPO) technology has the followingadvantages: non-framework titanium is less, which is beneficial toreduce the decomposition of the hydrogen peroxide by the non-frameworktitanium; the grain size is small and the porous channel is short, whichis beneficial to reduce the resistance effect of the ten-membered ringmicropores on the diffusion of reactants and products in the liquidphase reaction. However, those skilled in the art know that thesynthesis of TS-1 by the classical method requires the use of highpurity templating agent TPAOH and silicon ester and titanium ester rawmaterials to prevent the introduction of impurity metal ions, especiallysodium, potassium and other alkali metal ions. It is found that, even ifa very small amount of alkali metal ions present in the hydrothermalsynthesis of TS-1, the amount of titanium entering the zeolite frameworkwill be obviously reduced, thereby obviously reducing the liquid phaseoxidation activity of the catalyst. J. Catal., 1995, 151, 77-86 hassystematically shown the influence of sodium ion content on the TS-1zeolite by adding sodium salt to the synthetic gel of TS-1. The dataprovided by the literature show that when the sodium ion content in thegel is too high (Na/Si molar ratio≥0.05), the synthesized TS-1 zeolitehas no catalytic activity for n-octane oxidation; and only when thesodium ion content in the gel is very low (Na/Si molar ratio≤0.01), thecatalytic activity of the synthesized TS-1 zeolite can close to a normallevel. The data provided by the literature also show that although highsodium content can obviously reduce or even completely deactivate theepoxidation catalytic activity of the TS-1 zeolite, it can significantlypromote the decomposition of hydrogen peroxide. Based on the results ofthe liquid phase oxidation reactions, the empirical value with themaximum allowable value of the alkali metal ions of 0.01 in the gelduring the hydrothermal synthesis of the TS-1 zeolite is suggested(indicated by the molar ratio of the alkali metal ions to Si atoms, seeJ. Catal., 1995, 151, 77-86; Stud. Surf. Sci. Catal., 1991, 69, 79-92;Stud. Surf. Sci. Catal., 1991, 60, 343-352; Appl. Catal. A-gen., 2000,200, 125-134; Front. Chem. Sci. Eng., 2014, 8, 149-155.). According tothis standard, the content of alkali metal ion impurities of manycommercial templating agent TPAOH solutions exceeds the standard.Therefore, the practice of adding cation exchange resin to the syntheticgel during hydrothermal synthesis of the TS-1 zeolite has been proposed.This is because cation exchange resins can capture alkali metal cationsin the synthetic gel through ion exchange (RSC Adv., 2016, DOI:10.1039/C5RA23871D.).

In the liquid phase epoxidation of propylene with methanol solvent (lowtemperature and high pressure reaction conditions), the main problem tobe solved when preparing the catalyst is to reduce as much as possiblethe negative influence of the microporous diffusion resistance on thereaction conversion rate. Therefore, the secondary hydrothermaltreatment of the TS-1 zeolite matrix by using aqueous solution of theTPAOH templating agent is a common knowledge in the area of themodification of the TS-1 zeolite matrix. In this treatment, partialdissolution-recrystallization process occurs on the TS-1 zeolite matrix.The method aims to use the partial dissolution-recrystallization processto produce mesopores or/and hollow cavities in the crystals of the TS-1zeolite and communicate with the micropores, so as to achieve thepurposes of further improving the microporous diffusion of the nano TS-1zeolite and enhancing the catalytic activity of the nano TS-1 catalystin the liquid phase epoxidation.

For example, Chinese invention patents (application numbers) 98101357.0,98117503.1, 99126289.1 and 01140182.6 disclose a modification method ofsecondary hydrothermal treatment for the TS-1 zeolite matrix withaqueous solution of organic bases, including quaternary ammonium base.Through the research by nitrogen physical adsorption, transmissionelectron microscopy, UV-visible diffuse reflectance spectroscopy andother methods, the publication literature Microporous and MesoporousMaterials 102 (2007) 80-85 shows that, the partialdissolution-recrystallization process truly occurs during the process ofhydrothermal modification of the TS-1 zeolite with the TPAOH solution.The process produces two direct modification effects on the TS-1zeolite: first, cavities are produced inside the crystals; and second,part of the non-framework titanium (guest titanium species, such asanatase) is converted as framework titanium. The former is helpful tothe reaction by shortening the length of the micropores and reducing themicroporous diffusion resistance of the TS-1 zeolite; and the latter isbeneficial to the reaction by removing the guest species in themicropores, unclogging the microporous channels, increasing the numberof active sites and reducing the number of titanium oxide species ofdecomposing hydrogen peroxide. In addition, Chinese invention patent(application number) 201010213605.0 also discloses a method of modifyingTS-1 with water vapor of organic base. The modification effect isdescribed by the phenol hydroxylation reaction (liquid phase) tosynthesize diphenol. The purpose of the invention is to overcome thedefect of large wastewater discharge of the hydrothermal modificationmethod which uses the organic base solution.

In addition to the above secondary hydrothermal modification (alsocalled partial dissolution-recrystallization modification) methodintended to solve the problem of microporous diffusion limitation, thefollowing literature also reports several titanium silicalite zeolitemodification methods aimed at the framework titanium active sites andtheir local environment.

For example, Chem. Eur. J. 2012, 18, 13854-13860 reports the researchwork of the first characterization of a new hexa-coordinate frameworktitanium active site (Ti(OSi)₂(OH)₂(H₂O)₂) using UV Raman spectroscopy.It involves modification of the TS-1 zeolite at 80° C. with a lowconcentration NH₄HF₂ solution containing a small amount of hydrogenperoxide. As a result, the isolated tetra-coordinate framework titaniumactive site (Ti(OSi)₄) in the conventional TS-1 zeolite is convertedinto isolated hexa-coordinate framework-like titanium active site(Ti(OSi)₂(OH)₂(H₂O)₂). The novel hexa-coordinate framework-like titaniumactive site produces characteristic absorption at 695 cm⁻¹ in the UVRaman spectrum, and exhibits good catalytic activity in the liquid phaseepoxidation reaction of propylene. It can be confirmed from thespectrogram supplied by the literature that, the modification methodonly converts part of the tetra-coordinate framework titanium activesite (the absorption position is at 960 cm⁻¹) into the framework-likeactive site, but does not change the local environment of the remainingtetra-coordinate framework titanium (the characteristic absorption at960 cm⁻¹ does not undergo a shift). In fact, the modification methodbased on hydrogen peroxide and NH₄HF₂ solution has been reported in thefollowing literature: Angew.Chem. 2003, 115, 5087-5090; Angew.Chem.Int.Ed. 2003, 42, 4937-4940; Adv. Synth.Catal.2007, 349, 979-986; Appl.Catal. A 2007, 327, 295-299. The TS-1 zeolite modified by the method canimprove the catalytic activity and the selectivity for the benzenehydroxylation reaction (liquid phase).

Phys. Chem. Chem. Phys., 2013, 15, 4930-4938 mentions a method forpreparing a framework fluorine-containing titanium silicalite zeolite(F—Ti—MWW) by treating layered titanium silicalite zeolite Ti—MWW(treatment temperature of 40-150° C.) with a 2 M nitric acid solutioncontaining NH₄F. When the fluorine atom is connected with the siliconadjacent to the framework titanium, that is, SiO_(3/2)F appears besidethe framework titanium, the local environment of the framework titaniumis changed. SiO_(3/2)F enhances the electropositivity of the titaniumsite through the electron-withdrawing effect, and the titanium site withthe enhanced electropositivity improves the epoxidation reactionactivity of olefins (liquid phase) by producing more electrophilicO^(α)(Ti—O^(α)—O^(β)—H). Chinese invention patent (application number)201210100532.3 also discloses a framework-containing-fluorine MWWzeolite, and its preparation and application methods. In addition, ACSCatal., 2011, 1, 901-907 illustrates that NH₄F modification can reducethe hydrophilic hydroxyl groups on the surface of the TS-1 zeolite,thereby enhancing the surface hydrophobicity of the catalyst.

Chem. Commun., 2016, 52, 8679 provides a modification method forhydrothermal treatment of TS-1 zeolite (170° C.) with a solutioncontaining both ethylamine and tetrapropylammonium bromide. The methodconverts part of the tetra-coordinate framework titanium (Ti(OSi)₄) intohexa-coordinate active titanium species (Ti(OSi)₂(OH)₂(H₂O)₂) which isstill linked to the framework. This method is a hydrothermal reactionprocess characterized by a forward reaction of selectively dissolvessilicon and a backward reaction of recrystallizing the dissolvedsilicon. This method has the same modification effect as theabovementioned modification with a low concentration NH₄HF₂ solutioncontaining a small amount of hydrogen peroxide. However, it is believedthat, compared with the F-containing method, the F-free method is easierto optimize the ratio of the tetra-coordinate and the hexa-coordinateactive titanium species in the TS-1 zeolite. Besides, the F-free methodcan avoid the problem of dissolving titanium by fluoride. Therefore, theF-free method has obvious advantages. Moreover, the modified TS-1zeolite exhibits the modification effects of pronouncedly improving theactivity and selectivity in the epoxidation of cyclohexene (liquidphase).

In addition, there are also a few reports on the ion modification methodof the titanium silicalite zeolite.

For example, Chinese invention patent (application number)201480052389.2 discloses a method for preparing a zinc-modified titaniumsilicalite zeolite catalyst, which involves the impregnation (100° C.)of TiMWW zeolite with an aqueous solution containing zinc acetate toobtain a ZnTiMWW zeolite with Zn content of 0.1-5 wt. % (1.6 wt. % inthe embodiment). Said method also involves such post treatment of theimpregnated sample as filtration, washing, and spray drying. TheZn-containing TiMWW zeolite can be used as catalyst for the synthesis ofpropylene oxide in the liquid phase propylene epoxidation system, but itneeds acetonitrile as solvent.

In addition, Applied Catalysis A: General 200 (2000) 125-134 reports thepractice of performing alkali metal ion exchange treatment on the TS-1zeolite by using the potassium carbonate solution at room temperature.As the sodium exchange effect on TS-1 zeolite with 1 M NaOH solutionreported by J. Catal., 1995, 151, 77-86, the potassium ion in the strongalkaline potassium carbonate solution can also conduct similar ionexchange reaction with the silicon hydroxyl near the framework titanium,which replaces the hydrogen ion on the silicon hydroxyl, therebychanging the local environment of the framework titanium and influencingthe infrared spectrum characteristics of the framework titanium.However, from the provided results of the liquid phase oxidationreaction of hexane and 2-hexene, the alkali metal ion exchange treatmentat room temperature reduces the catalytic activity of treated TS-1zeolite.

Because people have known in the TS-1 zeolite synthesis research thatthe presence of the alkali metal ions in the hydrothermal synthesissystem is unfavorable for the introduction of titanium into theframework of TS-1, and people have also discovered through ion exchangeresearch that the introduction of the alkali metal ions into the TS-1zeolite in the post-synthesis process is also unfavorable to the liquidphase oxidation reaction based on hydrogen peroxide oxidant, thereforepeople do not know the real catalytic application of the alkali metalion modified TS-1 zeolite so far.

Nevertheless, some literatures such as Catal. Lett., 8, 237 (1991) andStud. Surf. Sci. Catal., 84, 1853 (1994) have reported that the TS-1zeolite framework often contains very low content of trivalent metal ionimpurities (such as Al³⁺ and Fe³⁺ ) which may produce bridging hydroxylgroups with strong proton acidity. The very small amount of strong acidsites will cause acid-catalyzed side reactions in the liquid phaseoxidation reactions catalyzed by the TS-1 zeolite, thereby reducing theselectivity of the reaction. The introduction of a very low content ofalkali metal ions into such TS-1 zeolite can effectively prevent theacid sites from destroying the catalyst selectivity. However, in thiscase, the role of the very small content of alkali metal ions in TS-1zeolite is counter cation which neutralizs the zeolitic acid sites. Forthe liquid phase oxidation reaction, if the alkali metal ions introducedinto the TS-1 zeolite exceed the amount required to neutralize the acidsites, then side effects such as reduction of catalyst activity would becaused.

In addition, those who skilled in the art know that, variouslow-temperature selective oxidation reactions catalyzed by the titaniumsilicalite zeolite use aqueous hydrogen peroxide solution as oxidant.The commercial hydrogen peroxide solution often contains 200-300 ppmacid stabilizer (50 wt. % H₂O₂ has a pH value of about 1-2). The acidstabilizer enters the titanium silicalite zeolite catalytic reactionsystem together with hydrogen peroxide, which will acidify the reactionmedium (in the propylene epoxidation reaction medium, the pH value ofthe hydrogen peroxide-methanol feed (3 molH₂O₂/L) is about 3.0), andwill also reduce the reaction selectivity. In addition, when hydrogenperoxide molecules are activated on the titanium active site of thetitanium silicalite zeolite through a “five-membered ring” manner, atransient peroxy proton (Ti(η²)—O—O⁻H⁺) with strong acidity is produced.In order to neutralize these acidic substances and proven their negativeinfluence on the selectivity of the propylene epoxidation, many patentsadopt the strategy of adding alkaline substances to the reaction medium.For example, the alkaline additives mentioned in the Chinese inventionpatent (application number) 201410512811.x are ammonia, amine,quaternary ammonium base and M¹(OH)_(n). M¹ is alkali metal or alkalineearth metal. The additives mentioned in Chinese invention patent(application number) 00124315.2 are alkali metal hydroxide, alkali metalcarbonate and bicarbonate, alkali metal carboxylate and ammonia. Chineseinvention patent (application number) 03823414.9 claims the introductioninto the reaction medium of less than 100 wppm alkali metal and alkalineearth metal, or alkali and alkali cation with a pKB value of less than4.5, or the combination of alkali and alkali cation, wherein wppm isbased on the total weight of hydrogen peroxide in the reaction mixture.Chinese invention patent (application number) 201180067043.6 claims thatthe addition of 110-190 micromoles potassium cations and phosphorus withat least one hydroxy acid anion into the reaction medium. The micromoleslevel loading of additive is based on 1 mole of hydrogen peroxide in thefeed. Therefore, the existing literature tells us that the way to dealwith the extrinsic acidity of the catalyst is to add alkaline substancesto the reaction medium or mixture, including alkali metal hydroxides orweak acid salts that can release hydroxide ion via hydrolysis. Theloadings of the alkaline substances are generally determined based onthe amount of hydrogen peroxide in the feed. According to theinformation disclosed in the Chinese invention patent (applicationnumber) 201480052389.2, at least most of the alkaline substances addedto the reaction medium can flow out of the reactor outlet along with thereaction products.

However, Applied Catalysis A: General 218 (2001) 31-38 pointed out that,in a batchwise propylene liquid phase epoxidation test, the attempt ofintroducing a small amount of sodium carbonate into the reactant, inorder to increase the pH value of the reaction liquid, and consequentlyto further inhibit the side reaction between the epoxidation product andthe solvent, so as to enhance the selectivity of propylene oxide, caneasily cause the deactivation of the catalyst due to the accumulation ofsodium carbonate on the catalyst. The results in Table 5 in theliterature show that if the loading of Na₂O on TS-1 reaches 1.36 wt. %(approximately equivalent to Na/Si=0.027) by sodium carbonateimpregnation, the catalyst activity (hydrogen peroxide conversion rate)may be reduced by nearly a half.

It should be clarified that, the practice that lets the TS-1 zeolitecontain a very low amount of alkali metal ions for the purpose ofreplacing the bridging hydroxyl (protonic acid sites) produced by thevery low content of trivalent metal ion impurities (such as Al³+ andFe³⁺) on the zeolite framework, so as to prevent the oxidation productsfrom being further converted by acid-catalyzed side reactions, and thepractice that adds a small amount of alkaline substances to the reactionmedium, including the addition of alkali metal ions or hydroxidesthereof to neutralize the acidity of the reaction medium and thetransient peroxy protons produced by the activation of hydrogen peroxideby TS-1, are all not the same alkali metal ion modification as mentionedin the present invention.

In addition, we have already known that Chinese invention patent(application number) 200910131992.0 discloses a method for hydrothermalmodification of TS-1 using an aqueous solution of a mixture of organicbase and inorganic base. The inorganic base involves ammonium hydroxide,sodium hydroxide, potassium hydroxide and barium hydroxide; and theorganic base involves urea, quaternary ammonium base, fatty amine andalcohol amine compounds. Embodiments 2, 3, 4, 7, 8 and 11 of the patentrespectively relate to the use of sodium hydroxide and ethylenediamine,potassium hydroxide and TPAOH, the potassium hydroxide andtriethanolamine, sodium hydroxide and n-butylamine, the potassiumhydroxide and TPAOH, and a base combination of the sodium hydroxide andTPAOH. In the above embodiments, the temperatures of hydrothermalmodification are respectively 180° C., 150° C., 180° C., 120° C., 90° C.and 180° C. The invention uses the reaction of phenol hydroxylation todiphenol (liquid phase) and the reaction of cyclohexanone ammoniaoxidation (liquid phase) to demonstrate the comprehensive improvement ofactivity, selectivity and activity stability of the modified catalyst.However, it is worth noting that the invention uses Fourier TransformInfrared Spectroscopy (FT-IR) to confirm that the modified TS-1zeolites, including the modifications involved by the inorganic bases,show the same infrared absorption band of framework titanium at 960 cm⁻¹as the unmodified matrix. Therefore, the patent uses the ratio of theabsorption band intensity at 960 cm⁻¹ (I₉₆₀) to the absorption bandintensity at 550 cm⁻¹ (I₅₅₀) to characterize the influence of mixed basemodification on the relative content of the framework titanium. Chineseinvention patent (application number) 200910131993.5 discloses a methodfor hydrothermal modification of the TS-1 zeolite using apore-forming-agent containing aqueous solution of inorganic base and/ororganic base. The inorganic base involves ammonium hydroxide, sodiumhydroxide, potassium hydroxide and barium hydroxide. Embodiments 2 and 3respectively relate to a sodium hydroxide modification solutioncontaining starch and a potassium hydroxide modification solutioncontaining polypropylene. Similar to Chinese invention patent(application number) 200910131992.0, the invention also uses thereaction of phenol hydroxylation to diphenol (liquid phase) and thereaction of cyclohexanone ammonia oxidation (liquid phase) todemonstrate the comprehensive improvement of activity, selectivity andactivity stability of the modified catalyst. Moreover, the inventionalso uses Fourier Transform Infrared Spectroscopy (FT-IR) to confirmthat the modified TS-1 zeolites, including the modifications involved byinorganic base, show the same infrared absorption band of the frameworktitanium at 960 cm⁻¹ as the matrix. Therefore, the patent also uses theratio of absorption band intensityat 960 cm⁻¹ (I₉₆₀) to the absorptionband intensityat 550 cm⁻¹ (I₅₅₀) to characterize the influence of basemodification on the relative content of the framework titanium. Theabove invention does not mention whether the modified TS-1 zeolitecontains the alkali metal ions. The purpose of the invention is actuallyto improve the microporous diffusivity of the TS-1 zeolite throughhydrothermal modification. However, the characterization results ofFourier Transform Infrared Spectroscopy (FT-IR) provided by theinventions show that, the modified TS-1 zeolites, including themodifications involved by the inorganic base, show the infraredabsorption band of the framework titanium at 960 cm⁻¹, which is animportant feature. It indicates that the use of the modification methodprovided by the above inventions, the alkali metal ions do not affectthe asymmetric stretching vibration of the Si—O—Ti bond (or called theSi—O bond stretching vibration disturbed by the framework titanium). Inother words, the alkali metal ions do not affect and change the localenvironment of the framework titanium. The most reasonable explanationof this phenomenon is that most of the alkali metal ions that maycontain in the modified TS-1 zeolite have been eliminated by theabovementioned inventions via the generally adopted post-treatmentsteps, such as washing.

In addition, we also note that the following invention patents relate toalkaline solution treatment. However, the so-called alkaline solutiontreatment is not the same alkali metal ion modification as the one thatwill be provided by the present invention.

For example, the TS-1 moulding method disclosed in the Chinese inventionpatent (application number) 201010511572.8 relates to an alkalinesolution treatment step, and the catalyst prepared by the patentedmethod is used for propylene liquid phase epoxidation to producepropylene oxide. However, the patent has the following features,firstly, the hollow TS-1 zelite (“hollow” means TS-1 has been subjectedto a secondary hydrothermal modification. The method of the secondaryhydrothermal modification can be seen in Chinese invention CN1132699Cwith application number 99126289.1) is moulded by using a silica sol asbinder, said silica sol containing a kind of silane which has at leasttwo hydrolysable groups, to obtain a molded body. Then, the molded bodyis subjected to heat treatment by the alkaline solution involving thesodium hydroxide, the potassium hydroxide, the tetramethylammoniumhydroxide and tetraethylammonium hydroxide, and is dried and calcined toobtain the TS-1 catalyst which has enough anti-crushing strength andextra-high zeolite content. The temperature range of heat treatment is60-120° C.; the concentration of the used alkaline solution is withinthe range of 0.1-10 mol %; and the ratio of the alkaline solution to themolded body is (0.5-5)/l. It can be clarified that the alkaline solutionheat treatment involved in the invention is not used to modify themolded body, but is used to promote the hydrolysis reaction of silaneand/or siloxane in the binder, so that the molded body obtains enoughanti-crushing strength. This can be confirmed by the proposal inparagraph [0034] of the patent text (“the use amount of the alkali canbe selected according to the amount of silane and/or siloxane with atleast two hydrolyzable groups.”). In addition, according to the heattreatment temperature, time and index data, i.e., particle strength,hydrogen peroxide conversion rate and propylene oxide selectivity, inembodiments 1-7 for comparative analysis, the upper limit of the heattreatment temperature of 120° C. in the patent corresponds to the lowerlimit of the treatment time of 2 hours. Namely, in order to achieve agood molding effect, the temperature of the alkaline solution heattreatment should not be high and the time should not be long; otherwise,the reaction activity and selectivity of propylene liquid phaseepoxidation may be reduced. Alkaline solution heat treatment conditionsof 90° C. and 6 hours adopted in embodiment 4 are optimal values.

For another example, a preparation method of a high-performance titaniumsilicalite zeolite catalyst disclosed in the Chinese invention patent(application number) 201310146822.6 also relates to the step ofmodifying the micron-sized titanium silicalite zeolite with the alkalinesolution. The main feature of the preparation method is that themicron-sized titanium silicalite zeolite catalyst is first treated withthe alkaline solution for the purpose of manufacturing a large number ofmesopores and macropores on the micron-sized titanium silicalite zeoliteto improve the accessibility of the titanium active site inside thelarge-grained titanium silicalite zeolite and facilitate the diffusionand output of product molecules from the interior of large grains. Then,the titanium-containing modified solution is used to treat the titaniumsilicalite zeolite catalyst containing mesopores and macropores again,for the purpose of introducing more active sites to the surface of thezeolite catalyst through the crystallization process. It can beclarified that although alkali metal hydroxide is mentioned in thealkaline solution treatment step of the invention, the patent requiresthat the zeolite treated in the step needs to be washed to meet therequirement of neutral pH. This means that washing in the step must bevery adequate. In this way, even if the alkali metal hydroxide is usedto complete the modification treatment of the alkaline solution, thealkali metal ions may not be left on the catalyst. This is becausehydroxyl anions may be left if the alkali metal ions are left, so thatthe harsh requirement of neutral pH cannot be satisfied. The reason thatthe invention proposes such a high requirement for the step of washingis that the subsequent modification step is to introduce the frameworktitanium active site on the surface of the zeolite. Based on the abovebackground introduction, it is not difficult to understand that if thezeolite is allowed to contain a large amount of alkali metal ions in thefirst step of alkaline solution treatment of the invention, it willinevitably prevent titanium from effectively entering the framework andbecoming the active site in the second step of modification.

In summary, in order to develop environmentally benign oxidation processby exploiting the hydrogen peroxide green oxidant, almost all of theexisting patents and publication literatures follow the requirements ofa liquid phase reaction mode to invent the preparation methods of thetitanium silicalite zeolite catalyst. Generally, the liquid phaseoxidation reaction is conducted at low temperature. Under thiscondition, the self-decomposition reaction of hydrogen peroxide is veryslow. Thus, in the liquid phase reaction mode, the selectivity(utilization rate) of hydrogen peroxide for the selective oxidationreaction is higher. The main challenge of developing the titaniumsilicalite zeolite catalyst for the liquid phase reaction mode is how toimprove the low temperature reaction activity of the catalyst. Highresistance of micropores of the titanium silicalite zeolite for masstransfer and diffusion under the liquid phase reaction conditions is amajor factor that limits the low temperature activity of the catalyst.This is the reason why many patents and publication literatures focus onthe use of nano TS-1 zeolite synthesized by the classical method as thematrix of catalyst and conduct secondary hydrothermal recrystallizationtreatment for the matrix whenever possible (generally in the presence oftetrapropylammonium hydroxide) in order to produce appropriate mesoporesand cavities inside the ultrafine TS-1 zeolite crystals so that thecatalyst can have better catalytic performance.

Different from the previous inventions, the alkali metal ion modifiedTS-1 zeolite and the preparation method thereof provided by the presentinvention are specially targeted at the gas phase epoxidation ofpropylene and hydrogen peroxide. The gas phase epoxidation of propyleneis conducted without the participation of any solvent under normalpressure and temperature above 100° C. Under this condition, thereactants of propylene and hydrogen peroxide directly contact with eachother in the form of gas molecules, and penetrate through a catalyst bedtogether to conduct the epoxidation. It is not difficult to understandthat due to the change of the reaction phases and the conditions and theabsence of methanol solvent, the reaction mechanism of gas phaseepoxidation of the propylene is impossible the same as that of theliquid phase epoxidation; the active sites required are also differentfrom those of the liquid phase epoxidation; and the main challenges andthe main problems to be solved in the preparation and modification ofthe catalyst will consequently be different from those of the liquidphase epoxidation.

But compared with the known liquid phase epoxidation technology, it isclear that the gas phase epoxidation technology of propylene andhydrogen peroxide has huge potential advantages. This is the significantvalue of the present invention. It is well known that the liquid phaseepoxidation technology (HPPO) of propylene must use a large amount ofsolvents to ensure that the propylene (oily) and hydrogen peroxideaqueous solution can become a stable homogeneous phase throughliquid-liquid mixing, so that the epoxidation can be conducted safely.At present, commercial production lines of the HPPO technology usemethanol as the solvent. Methanol is easily available and very cheap,and the methanol solvent is believed to have additional promotion effectin hydrogen peroxide activation and propylene epoxidation, probably viathe formation of a so-called “five-membered ring” transition state withhydrogen peroxide molecules and the active site of the frameworktitanium. However, methanol solvent also brings big trouble to thepractical application of the HPPO technology. Firstly, methanol easilycauses solvolysis side reactions with the propylene oxide product, whichresults in high-boiling propylene glycol monomethyl ether and otherby-products. These by-products not only remarkably reduce theselectivity of the propylene oxide, but also increase the difficulty ofwastewater treatment. Secondly, the methanol solvent must be recycledand needs complex purifying treatment (including hydrogenation,rectification and resin adsorption) before recycled, this makes the HPPOtechnology complicated and investment and energy consumption increased.In addition, although the recycled methanol solvent will be treated by acomplex purification process, more than ten or even 20 or 30 kinds oftrace impurities (including fusel, aldehyde, ether, ester and acetals)are still difficult to be removed. These trace impurities will berecycled back to the reactor together with methanol solvent,consequently accelerate the deactivation of the epoxidation catalyst andgreatly shortens the service cycle and life of the catalyst. The gasphase epoxidation of propylene and hydrogen peroxide can thoroughlyavoid the above problems because no solvent is used, and thus has a verygood development potential.

Since 2002, we have been engaged in the research related to the gasphase epoxidation of propylene and hydrogen peroxide. First, we havedeveloped a dielectric barrier discharge plasma technology that candirectly synthesize high-purity gaseous hydrogen peroxide from themixture of hydrogen and oxygen, which has been documented by thefollowing literature: Chem. Commun., 2005, 1631-1633; Modern ChemicalIndustry, Vol. 26 Supplement, 2006, P194-197; AIChE J, 53: 3204-3209,2007; Advanced Technology of Electrical Engineering and Energy, Vol 28,2009, No.3, P73-76; Chin. J. Catal., 2010, 31: 1195-1199; CIESC JournalVol 63, 2012, No. 11, P3513-3518; Journal of Catalysis 288 (2012) 1-7;Angew. Chem. Int. Ed. 2013, 52, 8446-8449; AIChE J, 64: 981-992, 2018;Chinese invention patents (application numbers) 200310105210.9,200310105211.3 and 200310105212.8. We have realized an in-situcontinuous synthesis of hydrogen peroxide gas by using the plasmatechnology, and completed the first stage of research work of gas phaseepoxidation of propylene by 2007 (Zhou Juncheng. Direct synthesis ofhydrogen peroxide by hydrogen and oxygen plasma method and itsapplication in the gas phase epoxidation of propylene [D]. Dalian:Dalian University of Technology, 2007). Specifically, a speciallydesigned two-stage integrated reactor was used in the research work. Thefirst stage reactor was a dielectric barrier discharge (DBD) plasmareactor for providing continuous and stable gaseous hydrogen peroxidefeed for the epoxidation reaction stage by taking the mixture ofhydrogen and oxygen as raw material (the concentration of the oxygen inthe hydrogen is less than 6 v %). The second stage reactor was a gasphase epoxidation reactor of propylene and gaseous hydrogen peroxide, itcontained TS-1 zeolite particles. In the research, the results of thegas phase epoxidation reaction obtained at 90° C. and 1 atm were: about7% of propylene conversion rate, 93% of propylene oxide (PO)selectivity, and 0.24 kg PO kgPO kg TS-1⁻¹h⁻¹ propylene oxide yield.Later, we carried out a more comprehensive research work by use the sameepoxidation system and a micron-sized large-crystal TS-1 zeolite(unmodified) synthesized with a non-classical method (also known as thecheap method) as catalyst. The results published on Chin. J. Catal.,2010, 31: 1195-1199 indicate that at the reaction temperature of 110°C., the selectivity of the propylene oxide is increased to about 95%,and the yield of the propylene oxide is maintained to about 0.25 kg POkg TS-1⁻¹h⁻¹. The reaction activity of the catalyst is stable for atleast 36 h in the gas phase epoxidation. However, the epoxidationselectivity, i.e., the utilization rate, of the hydrogen peroxide isonly about 36%.

We have already noted that Klemm et al. also reported the research workof gas phase epoxidation of propylene in 2008 [Ind. Eng. Chem. Res.2008, 47, 2086-2090]. They used a special glass carburetor or amicro-channel falling-film evaporator and 50 wt % aqueous hydrogenperoxide solution to provide gaseous hydrogen peroxide raw material forthe gas phase epoxidation. The gas phase epoxidation reactor was amicro-channel reactor with TS-1 zeolite coated inside. The reactionresults obtained at 140° C. and 1 atm were: selectivity of propyleneoxide was larger than 90%, and the yield of propylene oxide was largerthan 1 kg PO kg_(TS-1) ⁻¹h⁻¹. However, the utilization rate of thehydrogen peroxide was only about 25%.

The above preliminary research work about the gas phase epoxidation ofpropylene indicates that in the absence of the methanol solvent, thedirect contact of propylene and hydrogen peroxide gas can effectivelyconduct the epoxidation on TS-1 zeolite, a considerable yield of thepropylene oxide product can be obtained. Moreover, the selectivity ofthe propylene oxide can be as high as about 90%, which is very close tothe result of liquid phase epoxidation. However, the utilization rate ofhydrogen peroxide at the normal feed ratio of propylene and hydrogenperoxide in the gas phase epoxidation reaction is very low, generallyfalls in the range of 20-40%. The value is much lower than that of theliquid phase epoxidation (which is generally 85-95%). The researches ofSu Ji et al. (Journal of Catalysis 288 (2012) 1-7) and Ferrandez et al.[Ind. Eng. Chem. Res. 2013, 52, 10126-10132] indicate that, the reasonwhy the utilization rate of the hydrogen peroxide in the gas phaseepoxidation is very low is that, the self-decomposition side reaction ofhydrogen peroxide and the main reaction of epoxidation are highlycompetitive at high temperature (e.g., 110-140° C.). The decompositionreaction of hydrogen peroxide (to produce water and oxygen) can occur onboth the material surface of the reactor and the surface of catalyst.

It is obvious that, the first challenge of the gas phase epoxidationtechnology of propylene is the severe problem of hydrogen peroxideself-decomposition at high temperature. The rapid decomposition ofhydrogen peroxide at high temperature not only reduces the utilizationrate of hydrogen peroxide and the conversion rate of propylene, but alsoproduces oxygen which easily makes the organic gas in the reactor systemand the downstream separation system have an explosive composition andthus increases the risk of explosion accidents.

In the previous invention patent applications, we have disclosed twoTS-1 zeolite modification methods which are mainly used for the gasphase epoxidation of propylene and hydrogen peroxide. The first methodis to treat the TS-1 zeolite using a mixed solution oftetrapropylammonium hydroxide (TPAOH) and inorganic salt (lithium,sodium, potassium and mixtures thereof) (Chinese invention patent(application number) 201110338224.x); and the second method is to treatthe TS-1 zeolite using a mixed solution of tetrapropyl quaternaryammonium cation halide and inorganic base (alkali metal hydroxides oflithium, sodium and potassium) (Chinese invention patent (applicationnumber) 201110338451.2 and U.S. Pat. No. 9,486,790B2). In the follow-upresearch, we realized that the two disclosed TS-1 zeolite modificationmethods in the abovementioned inventions have limitations in improvingthe gas phase epoxidation performance of propylene and hydrogen peroxide(the highest conversion rate of propylene for the modified TS-1 zeolitein the gas phase epoxidation reaction is ≤9%), this is because that, inthese inventions we pursue for the dual applicability of the catalyst inboth the liquid phase and gas phase epoxidations of propylene andhydrogen peroxide. Therefore, the two disclosed modification methodscannot lead to the TS-1 zeolite catalyst that is especially suitable forthe gas phase epoxidation of propylene and hydrogen peroxide. The aboveinvention patents have a common feature, which underlines that theresidual alkali metal ions in the modified TS-1 zeolite are notconducive to achieving the modification effect. Therefore, it clearlystates in the technical solutions that, during the post-treatments, theTS-1 zeolite hydrothermally treated with a mixed solution containing thealkali metal ions must be fully washed with deionized water and the pHvalue of the filtrate should be less than 9 (the embodiment points outthe pH is preferably 7).

SUMMARY

In order to further improve the technical level of the gas phaseepoxidation of propylene and hydrogen peroxide, the present inventionprovides an alkali metal ion modified titanium silicalite zeolitecapable of selectively promoting the epoxidation of propylene andhydrogen peroxide in a gas phase reaction without the participation ofany solvent and a preparation method thereof.

The core of the present invention is to perform a degree controlledhydrothermal treatment on the titanium silicalite zeolite TS-1 with analkali metal hydroxide solution. After the hydrothermal treatment,alkali metal cations must remain on the titanium silicalite zeolite, andat least part of the alkali metal cations are on the silicon hydroxylsnear the framework titanium in the form of counter cations to modify thelocal environment of the framework titanium. The connotation of thelocal environment includes at least the electron cloud distribution andgeometric spatial factors of the framework titanium. The alkali metalhydroxides are preferably sodium hydroxide and potassium hydroxide, andless preferably lithium hydroxide. After repeated researches, we aresurprised to find that the alkali metal ion modification has almost noeffect on the liquid phase epoxidation of propylene and hydrogenperoxide at low temperature with methanol as solvent, but has anunexpected improvement effect on the gas phase epoxidation of propyleneand hydrogen peroxide in the absence of solvent and at high temperature(which is generally higher than 100° C. under normal pressure). For thegas phase epoxidation of propylene and hydrogen peroxide, frameworktitanium active site modified with alkali metal ions enable the catalystto obviously inhibit the self-decomposition side-reaction of hydrogenperoxide at a normal propylene/hydrogen peroxide feed ratio and increasethe conversion rate of propylene, so as to increase the utilization rateof hydrogen peroxide and reduce the generation of oxygen, therebygreatly improving the economy and safety of the gas phase epoxidation.

The specific implementation solutions and embodiments of the presentinvention mainly focus on TS-1 zeolite. This is mainly because TS-1 isthe most popular representative in the family of the titanium silicalitezeolite. TS-1 is relatively easier to synthesize, it is widely reportedin the literature, and used widely in industry. At present, it is theTS-1 zeolite which is commercially used in the HPPO technology of theliquid phase epoxidation of propylene and hydrogen peroxide.

In the alkali metal ion modified TS-1 zeolite prepared by the presentinvention, the framework titanium active site modified by the alkalimetal ions has unique infrared spectral characteristics, and thecharacteristic absorption peak of the vibrational spectrum appears in arange above 960 cm⁻¹ and below 980 cm⁻¹, which is a novel frameworktitanium active site different from the known tetra-coordinate frameworktitanium active site (infrared absorption is at 960 cm⁻¹) and thehexa-coordinate framework-like titanium active site(Ti(OSi)₂(OH)₂(H₂O)₂) (UV Raman absorption peak is at 695 cm⁻¹). Theliterature (J. Catal., 1995, 151, 77-86) has reported that the sodiumexchange of TS-1 with 1 M NaOH solution at 25° C. can also change theinfrared spectral characteristics of TS-1 zeolite framework titanium(the absorption peak at 960 cm⁻¹ is shifted to a shoulder peak at 985cm⁻¹), and that the sodium exchange conducted in the strong alkalinesolution is also the exchange reaction between the sodium ion in thesodium hydroxide and the hydrogen proton on the silicon hydroxyl nearthe framework titanium (NaOH+Si—OH=Si—O⁻¹ Na⁺+H₂O), the consequence ofthe exchange reaction is that the sodium ion exists on the siliconhydroxyl near the framework titanium in the form of counter cation, andnaturally changes the local environment of the framework titanium.However, in the present invention, the infrared characteristicabsorption of the framework titanium active site modified by the alkalimetal ion appears in the range above 960 cm⁻¹ and below 980 cm⁻¹, whichis different from the value (985 cm⁻¹) reported in the literature by atleast 5 wave numbers (cm⁻¹). It is not difficult for people familiarwith infrared spectroscopy to understand that two infrared absorptionswith such a large difference in the wave numbers should belong to thevibration of different framework titanium sites. In addition, it will beseen from the reference embodiment provided by the present inventionthat the sodium exchange TS-1 zeolite prepared according to the methodreported in J. Catal., 1995, 151, 77-86 (the framework titanium infraredcharacteristic absorption appears near 985 cm⁻¹, which is consistentwith the literature report) is basically inactive for the gas phaseepoxidation of propylene and hydrogen peroxide. In sharp contrast tothis, the sodium ion modified TS-1 zeolite obtained according to thepresent invention (the infrared characteristic absorption of theframework titanium appears in the range above 960 cm⁻¹ and below 980cm⁻¹) has high activity and high selectivity for the gas phaseepoxidation. People familiar with the titanium silicalite zeolite knowthat four framework silicons are first adjacent to the frameworktitanium. The four framework silicons are not equivalent in space, have16 possible hydroxyl positions. The relative positions of these siliconhydroxyls are quite different relative to the framework titanium site.We conclude from the study of quantum chemistry calculation that thedifference of infrared spectral characteristics of the frameworktitanium of the alkali metal ion modified TS-1 zeolite provided by thepresent invention from the room temperature sodium exchange TS-1 zeolitereported in the literature, should be caused by the different siliconhydroxyl positions occupied by the alkali metal countercations.

Briefly speaking, the present invention discloses a method which tellshow to use the alkali metal hydroxide solution to hydrothermally treat azeolite of titanium silicalite with controlled degree. The phrase“controlled degree” is mainly used to explain that the hydrothermaltreatment method provided by the present invention can ensure that thealkali metal ion is always located in the most favorable siliconhydroxyl position, so as to most favorably modify the local environmentof the framework titanium site of the modified TS-1 zeolite, therebymore effectively promote the gas phase epoxidation of propylene andhydrogen peroxide.

Therefore, in general, the present invention has the following features:the hydrothermal treatment of the TS-1 zeolite by the alkali metalhydroxide solution is carefully controlled, and the alkali metal cationsmust remain in the TS-1 zeolite after the hydrothermal treatment, and atleast part of the alkali metal cations are on the proper siliconhydroxyls near the framework titanium in the form of counter cations toappropriately modify the local environment of the framework titanium.Moreover, the framework titanium active site modified by the alkalimetal ions produces infrared characteristic absorption above 960 cm⁻¹and below 980 cm⁻¹.

To realize the degree controlled hydrothermal treatment in the presentinvention, firstly, a low-concentration alkali metal hydroxide solutionis selected for the hydrothermal treatment. Secondly, when TS-1 zeoliteis subjected to the hydrothermal treatment with the low-concentrationalkali metal hydroxide solution, proper hydrothermal treatmenttemperature, time and liquid-solid ratio must also be adopted. In otherwords, the concentration of the alkali metal hydroxide solution, thetemperature and time of the hydrothermal treatment, and the liquid-solidratio are the basic parameters that control the degree of thehydrothermal treatment.

The present invention requires the use of a micron size TS-1 titaniumsilicalite zeolite or small-crystal TS-1 zeolite synthesized by anon-classical method as matrix. Or more precisely, the present inventionis applicable to larger crystal size TS-1 zeolites with a single grainsize more than 0.3 micron or preferably with a single grain size morethan 0.5 micron. We are surprised to find that the cheap TS-1 zeolitesynthesized by the non-classical method has excellent catalyticperformance for the gas phase epoxidation of propylene and hydrogenperoxide after being modified by the method of the present invention. Insharp contrast to this, nano TS-1 zeolite (the aggregate size isgenerally below 200-300 nanometers) synthesized by the classical methodintroduced by Taramasso et al. (U.S. Pat. No. 4,410,501. 1983) orThangaraj et al. (J Chem Soc Chem Commun, 1992: 123) is not obviouslyimproved in performance of the gas phase epoxidation of propylene andhydrogen peroxide after being modified by the method of the presentinvention, and the performance index is far inferior to that of themodified cheap TS-1 zeolite. The reason for the difference is that thehydrothermal modification method of the alkali metal hydroxide solutionprovided by the present invention is fundamentally a dissolutionmodification, thus the TS-1 zeolite with too small crystal size islikely to be dissolved excessively or even thoroughly in themodification, so that the required alkali metal modified frameworktitanium active site cannot be produced.

As described above, raw materials for the synthesis of TS-1 zeolite bythe classical method have the features that the tetrapropylammoniumhydroxide is used as the templating agent, and silicon ester andtitanium ester are used as a silicon source and a titanium source,respectively. The morphology of the product observed on an electronmicroscope is characterized by irregular aggregates; the particle sizeof the aggregates is generally 200-300 nanometers; and the grain size ofthe primary crystals which form the aggregates is generally below 100nanometers. Although later people have done a lot of meaningfulimprovement work on the basis of Taramasso et al. and Thangaraj et al.,the above basic features of TS-1 synthesized by the classical method arenot changed and it is easy to judge. Because the cost of the rawmaterials of the classical method for synthesizing TS-1 is high, it isnot bad that the present invention cannot be applied to the superfineTS-1 synthesized by the classical method.

Those skilled in the art know that the cheap TS-1 can be synthesized bydifferent methods. For example, the following literature has reportedthe hydrothermal synthesis methods of the cheap TS-1: Zeolites andRelated Microporous Maierials: State of the Art 1994, Studies in SurfaceScience and Catalysis, Vol. 84; Zeolites 16: 108-117, 1996; Zeolites 19:246-252, 1997; Applied Catalysis A: General 185 (1999) 11-18; CatalysisToday 74 (2002) 65-75; Ind. Eng. Chem. Res. 2011, 50, 8485-8491;Microporous and Mesoporous Materials 162 (2012) 105-114; Chineseinvention patents (application numbers) 201110295555.x and201110295596.9. The raw materials of the cheap synthesis technology arecharacterized by using the tetrapropylammonium bromide as the templatingagent, and using ammonium hydroxide or organic amines such asmethylamine, ethylamine, ethylenediamine, diethylamine, n-butylamine andhexamethylenediamine as an alkali source. Silica sol and titaniumtetrachloride are mainly used as the silicon source and the titaniumsource, and sometimes titanium ester is also used as the titaniumsource. The morphology of the product observed on the electronmicroscope is characterized by monodisperse crystals with regularcrystal edges and planes, including large-grained thin plate crystals ofseveral microns, or coffin-shaped small-grained crystals of 300-600nanometers. For engineers familiar with the field, these characteristicsare also easy to identify.

In fact, with regard to the application area of titanium silicalitezeolite matrix in the present invention, it is better not to define itwith the terms of the classical synthesis method and the cheap synthesismethod, but with the term of the crystal size of TS-1 zeolite. This isbecause the zeolite of titanium silicalite can also be synthesized by agas-solid isomorphous substitution method, as described in theliterature of Ind. Eng. Chem. Res. 2010, 49, 2194-2199. Therefore, it isemphasized herein that the fundamental requirement of the presentinvention for the TS-1 zeolite matrix is that the grain size (referringto the single crystal rather than the aggregate) is at least ≥300nanometers, and preferably ≥500 nanometers; the synthesis method belongsto the cheap method or not is not a matter. However, in view of thecost, the TS-1 zeolite hydrothermally synthesized by the cheap methodmay be a preferred option.

In addition to the crystal size, the present invention also requires thetitanium silicalite matrix to have a relatively low silicon-titaniumratio and minimal non-framework titanium content. The two requirementsare easy to be understood. Firstly, the gas phase epoxidation ofpropylene and hydrogen peroxide requires the zeolite of titaniumsilicalite to have high-density titanium active sites, which isbeneficial to avoid the ineffective thermal decomposition of hydrogenperoxide. Secondly, as mentioned above, the hydrothermal modificationmethod of the alkali metal hydroxide solution provided by the presentinvention is essentially a controlled dissolution modification method,does not have the function of recrystallizing the dissolved substancesonto the zeolite framework, and certainly does not have the effect ofrecrystallizing the non-framework titanium species in the modifiedmatrix onto the framework. Therefore, if the matrix of the titaniumsilicalite zeolite contains too much non-framework titanium species,even though the total silicon-titanium ratio seems appropriate, it isnot conducive for the alkali metal ions modification to enrich thesurface layer of the modified titanium silicalite zeolite withsufficient framework titanium active sites.

In addition, the present invention also requires the matrix of thetitanium silicalite zeolite to have high enough relative crystallinity.This is not difficult to understand. After all, the crystal framework isthe support of the framework titanium.

The technical solution of the present invention is:

A kind of alkali metal ion modified titanium silicalite zeolite for gasphase epoxidation of propylene and hydrogen peroxide is provided. In thealkali metal ion modified titanium silicalite zeolite, the alkali metalions are reserved on the silicon hydroxyls of the modified TS-1 zeolite;an infrared characteristic absorption band of a framework titaniumactive site modified by the alkali metal ion is in a range above 960cm⁻¹ and below 980 cm⁻¹; a TS-1 zeolite matrix of the alkali metal ionmodified titanium silicalite zeolite meets the following requirements:the crystal size is ≥0.3 micron; a silicon-titanium molar ratio is ≤200;an index value of the framework titanium content is ≥0.40; and relativecrystallinity is ≥85%.

Further, the crystal size of the TS-1 zeolite matrix is ≥0.5 micron; thesilicon-titanium molar ratio is ≤100; the index value of the frameworktitanium content is ≥0.45; and the relative crystallinity is ≥90%.

The crystal size can be measured by a scanning electron microscope (SEM)or a transmission electron microscope (TEM). Those skilled in the artcan obtain the electron microscope images of a TS-1 zeolite sample to bemeasured according to electron microscope sample preparation andexperimental methods reported in any publication literature, and judgewhether the crystal size of the TS-1 zeolite matrix meets therequirements according to the electron microscope images. It should benoted that the crystal size in the present invention refers to the grainsize of the primary crystals (single crystals) of the TS-1 zeolite,rather than the size of the aggregates of the TS-1 zeolite. Thesuperfine TS-1 zeolite synthesized by the classical method often hasagglomerate size close to or even larger than 0.3 micron (300nanometers), but the grain size of the primary crystals (singlecrystals) is often less than 0.1 micron (100 nanometers), so thesuperfine TS-1 zeolite is not suitable for use in the present invention.Those skilled in the art can judge the TS-1 zeolite particles on theelectron microscope images are the single crystals or the aggregatesaccording to the following experience: generally, the single crystals ofthe TS-1 zeolite have very regular coffin-shaped crystal morphology orthin plate-shaped crystal morphology, while the agglomerates of thesuperfine crystals of the TS-1 zeolite are often irregularly spherical.

The silicon-titanium molar ratio refers to the total averagesilicon-titanium ratio of bulk phase of the sample. X-ray fluorescencespectroscopy (XRF) can be used with standard sample to obtainsilicon-titanium molar ratio data. Those skilled in the art can measurein person or entrust others to measure the silicon-titanium ratio dataof the TS-1 zeolite matrix according to the instructions of an XRFinstrument.

The index value of the framework titanium content is defined asI_(960cm−1)/I_(550cm−1), that is, the ratio of the absorption peakintensity of Ti—O—Si antisymmetric stretching vibration characterized at960 cm⁻¹ on the framework vibration infrared spectrum of the TS-1zeolite to the absorption peak intensity of the five-membered ringvibration of MFI structure characterized at 550 cm⁻¹. Those skilled inthe art know that the ratio has been generally accepted by researchersin the field and used to reflect the relative amount of the frameworktitanium in the TS-1 zeolite (for example, CATAL. REV.-SCI. ENG, 39(3).209-251 (1997) uses the value of I_(960cm−1)/I_(550cm−1) to give thecorrelation diagram P217 FIG. 4 b ). The larger the value ofI_(960cm−1)/I_(550cm−1) is, the higher the content of the frameworktitanium in the TS-1 framework is. Those skilled in the art can refer toexperimental method of the infrared vibration spectroscopy of thetitanium silicalite zeolite framework introduced by any publicationliterature to obtain the value of I_(960cm−1)/I_(550cm−1). The presentinvention provides the following practice for reference: pre-drying thespectral purity KBr at 110° C. for 4 hours, then mixing and grinding KBrand the TS-1 zeolite into a powder at a ratio of 100 to 200:1, pressingthe powder into a wafer at a pressure of 6 MPa, and putting the waferinto an infrared sample cell for testing. The peak intensity of the twoabsorption peaks at 960 cm⁻¹ and 550 cm⁻¹ can be directly obtained fromthe spectrum with the software of a spectrometer, so that the value ofI_(960cm−1)/I_(550cm−1) can be conveniently calculated.

The relative crystallinity refers to the ratio (expressed by percentage)of the sum of the intensity of five characteristic diffraction peaks(2θ=7.8°, 8.8°, 23.0°, 23.9° and 24.3°) of the TS-1 zeolite matrixmeasured by the X-ray powder diffraction method (XRD) to that of areference sample. Those skilled in the art can obtain the XRD patternsof the TS-1 zeolite matrix and the reference sample according to the XRDexperimental methods reported in any publication literature. The presentinvention recommends using embodiment 1 in the Chinese invention patent(application number) 201110295555.x to prepare the reference sample.Specifically: 220 ml of deionized water is added to 225 g of silica sol(20% wt); after stirring for 10 minutes, 18.4 g of tetrapropylammoniumbromide and 5.1 g seed crystals are added to the diluted silica sol;after continuing stirring for 20 minutes, a silicon solution isobtained; tetrabutyl titanate and acetylacetone are mixed at a massratio of 1:0.8, and stirred for 15 minutes to prepare a titaniumsolution; 19.7 ml of the prepared titanium solution is added to thesilicon solution; after stirring for 30 minutes, 57 ml of n-butylamineis added and continuously stirred for 15 minutes to obtain uniform gel;then 6.0 g of Na₂SO₄ is added and stirred for 10 minutes; then theobtained gel is added to a 2 l stainless steel autoclave reactor andcrystallized under autogenous pressure and 170° C. for 24 hours; aftercrystallization the product is filtered, washed to be neutral, and driedat 110° C. It is emphasized herein that before measuring the XRDpatterns of the TS-1 matrix and the reference samples, the two samplesto be measured must be calcined to ensure that the organic templatingagents in the samples are removed completely and more than 95%,preferably more than 98% of dry basis content of the zeolite isachieved. Thus, it is recommended to dry about 2 g of TS-1 matrix andabout 2 g of reference sample overnight at 110° C., and then place thesamples in a muffle furnace for temperature-programmed calcination. Thetemperature-programmed calcination starts at room temperature, and thetemperature is raised to 300° C. at a temperature rise rate of 10°C./min, and then the temperature is raised from 300° C. to 500° C. at atemperature rise rate of 1° C./min and kept constant until the sample iscompletely white.

The TS-1 zeolite which satisfies the above indexes can be used as themodified matrix of the present invention. The TS-1 zeolite suitable forthe present invention can be purchased from the market, or can besynthesized by engineers familiar with the field according to therelevant publication literature and patent documents. If the TS-1zeolite is synthesized by the engineers, the present inventionrecommends adopting the hydrothermal synthesis method of the TS-1zeolite reported in the following publication literature and patentdocuments: Zeolites and Related Microporous Maierials: State of the Art1994, Studies in Surface Science and Catalysis, Vol. 84; Zeolites 16:108-117, 1996; Zeolites 19: 246-252, 1997; Applied Catalysis A: General185 (1999) 11-18; Catalysis Today 74 (2002) 65-75; Ind. Eng. Chem. Res.2011, 50, 8485-8491; Microporous and Mesoporous Materials 162 (2012)105-114; Chinese invention patents (application numbers) 201110295555.xand 201110295596.9.

A qualified TS-1 zeolite matrix must remove the organic templating agentbefore modification. It is the common knowledge in the art to remove theorganic templating agent from a zeolite. The present invention providesthe following reference practice: drying an appropriate amount of TS-1zeolite matrix overnight at 110° C., and then placing the sample in themuffle furnace for temperature-programmed calcination. Thetemperature-programmed calcination starts at room temperature, and thetemperature is raised to 300° C. at a temperature rise rate of 5°C./min, and then the temperature is raised from 300° C. to 400° C. at atemperature rise rate of 1° C./min and kept constant for 12 hours; thenthe temperature is raised to 450° C. at the same temperature rise rateand kept constant for 12 hours; and finally, the temperature is raisedto 500° C. at the same temperature rise rate and kept constant until thesample is completely white.

The preparation method of the alkali metal ion modified titaniumsilicalite zeolite for the gas phase epoxidation of propylene andhydrogen peroxide comprises the following steps:

At first step: preparing an alkali metal hydroxide modificationsolution. In order to achieve the effect of degree controlledhydrothermal modification, the present invention requires that:

The preferred range of the concentration of the alkali metal hydroxidesolution is: a lower limit of 0.05 mol/L and an upper limit of 0.2 mol/L(calibrated at room temperature), and a more preferred range is: thelower limit of 0.08 mol/L and the upper limit of 0.15 mol/L (calibratedat room temperature).

The alkali metal hydroxide is preferably lithium hydroxide, sodiumhydroxide and potassium hydroxide; and more preferably sodium hydroxideand potassium hydroxide.

When the modification solution is prepared, any one of the alkali metalhydroxides recommended above can be used alone, or a mixture of any twoof the alkali metal hydroxides in any ratio can be used, or a mixture ofthree hydroxides in any ratio can also be used. When more than twoalkali metal hydroxides are used to prepare the modified solution, thesolution concentration refers to the sum of the molar concentrations ofvarious hydroxides.

Considering that commercially available alkali metal hydroxides containa certain amount of impurities, the purity of the raw materials of thealkali metal hydroxides should be analyzed by chemical titration beforethe modified solution is prepared. Those skilled in the art can performthe titration operation according to a conventional chemical analysismethod. Similarly, after the modified solution is prepared, the sameconventional chemical analysis method should be used to calibrate thehydroxide concentration of the modification solution. Because the alkalimetal hydroxides release a large amount of heat in the dissolutionprocess, the concentration is calibrated only when the solution iscooled to room temperature.

During modification, the concentration of the alkali metal hydroxide inthe modification solution will decrease as a result of the reaction ofthe alkali solution with the zeolite framework (for example,NaOH+Si—OH=Si—O⁻ Na^(++H) ₂O). Meanwhile, the used alkali metalhydroxide solution will also contain low concentrations of silicate,titanate and silicotitanate due to the framework dissolution anddesilication reaction (a small amount of framework titanium isinevitably dissolved in the process). However, undoubtedly, the usedalkali metal hydroxidesolution can be recycled conditionally, which canreduce the modification cost and waste solution discharge. Before theused solution is recycled, the concentration of the alkali metalhydroxide needs to be accurately measured in order to restore theinitial concentration by supplementing the alkali metal hydroxide; andthe concentrations of the silicate, the titanate and the silicotitanatecontained in the solution also need to be measured in order to controlthe number of cycles. In order to avoid too complicated and lengthydescription of the present invention and to make it easier for thecolleagues to understand the gist of the present invention, the presentinvention will not provide detailed illustration for the recycling ofthe used modification solution herein and in subsequent embodiments. Theengineers in the art can recycle the used modification solutionaccording to the common sense.

However, it should be stated that, just as the used modificationsolution can be recycled, it is beneficial that a certain amount ofalkali metal silicate, titanate and titanium silicate is addeddeliberately into the fresh modification solution in order to controlthe degree of dissolution modification. We have even found that theintroduction of suitable amount of alkali metal carbonate andbicarbonate into the fresh modification solution can also assist incontrolling the degree of dissolution modification. The commonality ofthe above salts is that they belong to strong alkali and weak acidsalts, and can be hydrolyzed in the aqueous solution to provide alkalimetal cations and hydroxyl anions, that is, alkali metal hydroxides areactually produced by hydrolysis. The difference from the direct additionof the alkali metal hydroxides is that the weak acid salt produces weakacid after hydrolysis, which can adjust the pH value of the modificationsolution to a certain extent, thereby helping to control the degree ofdissolution modification. In terms of the strong alkali and weak acidsalts, alkali metal phosphates and hydrogen phosphates should also becandidates, but the phosphates and the hydrogen phosphates are easy toaccumulate in the recycling of the modification solution, which is notconducive to multiple recycling of the modified solution. The engineersfamiliar with the field can select other strong alkali and weak acidsalts with alkali metal for the present modification purpose under theguidance of the above principles described in the present invention, andwill not be repeated.

At second step: conducting degree controlled hydrothermal treatment onthe TS-1 zeolite matrix by using the alkali metal hydroxide solution.The hydrothermal treatment can be conducted under static and stirringstates. In order to achieve the effect of degree controlled hydrothermaltreatment, the present invention requires that:

The preferred ratio range of volume of the modification solution toweight of the titanium silicalite zeolite matrix is from the lower limitof 5 ml/g to the upper limit of 15 ml/g, and a more preferably ratiorange is from the lower limit of 8 ml/g to the upper limit of 12 ml/g.

The preferred range of the hydrothermal modification temperature is fromthe lower limit of 100° C. to the upper limit of 200° C., and a morepreferred range is from the lower limit of 150° C. to the upper limit of190° C.

The preferable range of the hydrothermal modification time from thelower limit of 10 hours to the upper limit of 20 hours, and a morepreferable range is from the lower limit of 15 hours to the upper limitof 20 hours.

It should be specially explained that, in order to achieve the effect ofdegree controlled hydrothermal treatment, all the parameters such as theconcentration of the alkali metal hydroxide modification solution, theratio of volume of the modification solution to weight of the titaniumsilicalite zeolite matrix, the modification temperature and time need tobe taken into consideration together. It is not difficult for thoseskilled in the art to understand that the lower limit values of all thehydrothermal modification parameters involved in the first step and thesecond step can produce the weakest degree of dissolution modificationeffect, and the upper limit values of all the hydrothermal modificationparameters can produce the strongest degree of dissolution modificationeffect. Therefore, the modification conditions defined by thecombination of the lower limit values of all the parameters inevitablygenerate the lowest degree of dissolution modification result, and themodification conditions defined by the combination of the upper limitvalues of all the parameters inevitably generate the highest degree ofdissolution modification result. Therefore, it is not difficult tounderstand that the hydrothermal modification result between the lowestdegree and the highest degree will be obtained by selecting the lowerlimit value of a certain parameter and the intermediate values and theupper limit values of other parameters. Of course, it is not difficultto understand that the modification conditions formed by the combinationof different values of various parameters can generate different degreesof hydrothermal modification results. When the values of the parametersare designed for a given titanium silicalite zeolite matrix to reach adesired modification degree which guarantees a satisfactory catalyticperformance in the gas phase epoxidation of propylene and hydrogenperoxide, it is the exact meaning of the degree controlled hydrothermalmodification in the present invention. Clearly, the lowest degree ofdissolution modification effect and the highest degree of dissolutionmodification effect herein shall not be mistakenly understood as theworst modification effect and the best modification effect. Sometitanium silicalite zeolite matrices need the lowest degree ofhydrothermal dissolution modification, while some titanium silicalitezeolite matrices need higher degree of hydrothermal dissolutionmodification. Therefore, the present invention clarifies that thecombination conditions for the hydrothermal modification for a specifictitanium silicalite zeolite matrix should be determined throughexperiments, and shall be judged based on the position of the infraredvibration absorption peak of the active site, especially based on thegas phase epoxidation reaction data of propylene and hydrogen peroxide.

The engineers in the field can determine specific modificationconditions suitable for the specified titanium silicalite zeolite withinthe parameter value range recommended by the present invention accordingto specific considerations such as equipment use efficiency,modification cost and wastewater discharge.

At third step: conducting post-treatment on the hydrothermally modifiedTS-1 zeolite. Specifically, the third step comprises conventionalsolid-liquid separation, washing, drying and calcining steps. However,for the present invention, correct washing of the wet material of thezeolite after solid-liquid separation is the key. The present inventionrecommends using a low-concentration alkali metal hydroxide solution towash the wet material of the modified zeolite obtained by solid-liquidseparation, and the degree of washing is satisfactory when noprecipitate appears after the washing solution is neutralized with acid.The present invention requires that the preferred range of theconcentration of the alkali metal hydroxide solution used for thewashing purpose is from the lower limit of 0.001 mol/l to the upperlimit of 0.05 mol/l (calibrated at room temperature), and a morepreferred range is from the lower limit of 0.005 mol/l to the upperlimit of 0.04 mol/l (calibrated at room temperature). A furtherpreferred range is from the lower limit of 0.005 mol/l to the upperlimit of 0.03 mol/l (calibrated at room temperature). The alkali metalhydroxide is preferably lithium hydroxide, sodium hydroxide andpotassium hydroxide; and more preferably sodium hydroxide and potassiumhydroxide.

The necessity of washing is that, on one hand, the wet material of thezeolite obtained from solid-liquid separation still has a considerableamount of residual used modification solution existed in the form of asurface liquid film and a capillary condensate, roughly account for40-50 wt. % of the weight of the wet material. The residual usedmodification solution mainly contains free alkali metal hydroxides, italso contains silicate ions, titanate ions and titanium silicate ionsand larger zeolite framework fragments dissolved from the zeoliteframework. Excessive free alkali may continue to react with the siliconhydroxyl in the drying process and destroy an expected modificationdegree, while other species may become blockages of the porous channelsof the zeolite and even become the active sites that cause various sidereactions. On the other hand, improper washing method and degree mayeasily lead to the loss of useful alkali metal ions balanced on thesilicon hydroxyl. Therefore, it is very important to select correctwashing method and degree. The alkali metal hydroxide solution used forthe washing purpose in the present invention is an alkali metalhydroxide solution having a concentration much lower than that of themodified solution. The simplest practice is to use a lower-concentrationalkali metal hydroxide solution with the same type as the modificationsolution as the washing solution. The present invention provides thefollowing reasons for selecting the alkali metal hydroxide as thewashing solution: firstly, the use of the alkali metal hydroxidesolution as the washing solution is beneficial to supplement the lost ofthe useful alkali metal ions balanced on the silicon hydroxyls in thewashing process. Secondly, the alkali metal hydroxide solution isstrongly alkaline. The fall off and runoff of the useful alkali metalions balanced on the silicon hydroxyl can be prevented by maintainingthe strong alkalinity of the washing solution in the washing process.Otherwise, if deionized water is selected as the washing solution, theuseful alkali metal ions balanced on the silicon hydroxyl are easy tolose in the form of NaOH due to the reverse reaction of NaOH+Si—OH=Si—O⁻Na^(++H) _(x)O. That is, the hydrolysis reaction of strong alkali andweak acid salt (Si—O⁻ Na⁺). This is exactly the reason that the existingpatents involving inorganic base modification can remove the alkalimetal ions in the washing step. Therefore, in order to obtain the alkalimetal ion modified TS-1 zeolite, the deionized water is not suitable forthe present invention, and a solution with an acidic pH value alsocannot be used as the washing solution for the wet material aftersolid-liquid separation. The truth is self-evident.

Of course, it is entirely feasible in principle, but is not desirablefrom the perspective of environmental protection and cost thatlow-concentration quaternary ammonium alkali solutions or other organicalkali aqueous solutions with strong alkalinity are used as the washingsolution.

In addition, the purpose of the present invention emphasizing the use ofthe alkali metal hydroxide solution with a concentration lower than thatof the modification solution as the washing solution, is to reduce theresidual amount of free alkali metal hydroxide in the modified TS-1after washing. The shortcomings of excessive free alkali metal hydroxideremaining in the modified TS-1 has been explained above.

The simplest way for the solid-liquid separation in the presentinvention is centrifugation and filtration. However, when centrifugationand filtration are used, it is preferably not to introduce additivessuch as flocculants and filter aids to prevent the pH value of thesolution from changing or even causing the precipitate. Othersolid-liquid separation modes are allowed to be used as long as obviousliquid phase concentration, precipitation of precipitates and pH changeare not caused in the separation process.

The drying and calcining in the present invention can be conducted in anair atmosphere according to conventional practices. The referencepractice recommended by the present invention is as follows: the dryingtemperature range is 80-120° C., and the drying time is decided based onthe dry basis content of the sample not less than 90%. The recommendedfinal calcining temperature range is 400-550° C., and the constanttemperature time at the final temperature is not less than 3 hours.

The implementation effects of the present invention can be evaluated bythe following means:

Firstly, infrared spectroscopy is used to characterize the absorptionpeak position of the framework titanium of the modified TS-1 zeolite.The method comprises: putting an appropriate amount of sample from themodified TS-1 zeolite treated by the third step into a small beaker,putting an appropriate amount of spectral purity KBr into another smallbeaker, and putting the two small beakers into an oven at 110° C.simultaneously for pre-drying for 4 hours; then mixing and grinding KBrand TS-1 zeolite into powder at a ratio of 200:1, and pressing intowafers under a pressure of 6 MPa; putting the wafers into an infraredsample cell for testing to obtain an infrared spectra; and finally,using the second derivative spectrum in the infrared software toaccurately locate the infrared characteristic absorption peak positionof the framework titanium active site modified by the alkali metal ions.

Secondly, the X-ray fluorescence spectroscopy (XRF) method is used toobtain the silicon-titanium molar ratio and sodium ion content data ofthe modified sample.

In addition, a small fixed bed reactor is used to evaluate the gas phaseepoxidation performance of the modified TS-1 zeolite catalyst. It isrecommended to refer to the experimental devices and methods describedin the previously published journal papers and authorized Chineseinvention patents to evaluate the gas phase epoxidation of propylene andhydrogen peroxide. References recommended by the present inventioninclude: Chem. Commun., 2005, 1631-1633; Modern Chemical Industry, Vol.26 Supplement, 2006, P194-197; AIChE J, 53: 3204-3209, 2007; AdvancedTechnology of Electrical Engineering and Energy, Vol 28, 2009, No.3,P73-76; Chin. J. Catal., 2010, 31: 1195-1199; CIESC Journal Vol 63,2012, No. 11, P3513-3518; Journal of Catalysis 288 (2012) 1-7; Angew.Chem. Int. Ed. 2013, 52, 8446-8449; AIChE J, 64: 981-992, 2018; Chineseinvention patents (application numbers) 200310105210.9, 200310105211.3and 200310105212.8.

The characteristic of the evaluation method is the use of the integratedreactor. The upper segment of the integrated reactor is a self-coolingdielectric barrier discharge reactor for in-situ synthesis of gaseoushydrogen peroxide from hydrogen and oxygen plasma. The lower segment ofthe integrated reactor is a conventional fixed bed reactor whichcontains titanium silicalite zeolite particles (20-40 meshes) for thegas phase epoxidation of propylene and hydrogen peroxide. The workingprinciple of the integrated reactor is: hydrogen and oxygen are mixed at170 ml/min and 8 ml/min respectively under the control of a mass flowcontroller, and then enter the self-cooling dielectric barrier dischargereactor in the upper segment of the integrated reactor to synthesizegaseous hydrogen peroxide. The yield of the hydrogen peroxide is 0.35g/h. The synthesized hydrogen peroxide gas is carried by excess hydrogento enter the epoxidation reactor in the lower segment from a gas holebetween the two segments of reactors, and is fully mixed with thepropylene gas (18 ml/min) which enters the segment of reactor from aside line to jointly enter the TS-1 catalyst bed for the epoxidation.

The reaction conditions are: the loading of the TS-1 catalyst is 0.5 g(the catalyst powder is tabletted and then crushed and sieved to obtain20-40 meshes); the actual molar ratio of propylene to hydrogen peroxideis about 5:1; and the gas phase epoxidation is conducted at atmosphericpressure and 130° C.

The beneficial effects of the present invention are: the presentinvention uses the alkali metal hydroxide solution to perform the degreecontrolled hydrothermal treatment on the titanium silicalite zeoliteTS-1. After the hydrothermal treatment, alkali metal cations must remainin the titanium silicalite zeolite, and at least part of the alkalimetal cations are on the silicon hydroxyls near the framework titaniumin the form of counter cations to modify the local environment of theframework titanium. The connotation of the local environment includes atleast the electron cloud distribution and geometric spatial factors ofthe framework titanium. The alkali metal ion modification has anunexpected improvement effect on the gas phase epoxidation of propyleneand hydrogen peroxide in the absence of solvent and at high temperature(which is generally higher than 100° C. under normal pressure). For thegas phase epoxidation of propylene and hydrogen peroxide, themodification of the local environment of a framework titanium activesite with alkali metal ions enable the catalyst to obviously inhibit theself-decomposition side reaction of the hydrogen peroxide at a normalpropylene/hydrogen peroxide feed ratio and increase the conversion rateof the propylene, so as to increase the effective utilization rate ofthe hydrogen peroxide and reduce the generation of oxygen, therebygreatly improving the economy and the safety of the gas phaseepoxidation reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the framework vibration FT-IR spectra of catalyst samples ofembodiment 1 and reference embodiment 2.

FIG. 2 is the XRD patterns of catalyst samples of embodiment 2.

FIG. 3 is the framework vibration FT-IR spectra of catalyst samples ofembodiment 2.

FIG. 4 is the framework vibration FT-IR spectra of catalyst samples ofembodiment 3.

FIG. 5 is an SEM image of small-crystal TS-1 matrix adopted inembodiment 9.

DETAILED DESCRIPTION

The following embodiments only serve to further illustrate the presentinvention, but shall not be used to limit the contents of the presentinvention. The reagents and drugs involved in all the embodiments arecommercially available and analytically pure.

The SEM images are obtained with the NOVA NanoSEM 450 field emissionscanning electron microscope from American FEI Company. The voltage is230 kV, the frequency is 60 Hz, the current is 8 A, and themagnification is 800,000 to 1,600,000. Samples are dispersed intoanhydrous alcohol, and dripped on silicon wafers with a capillary. Then,after fixed on the conductive adhesive, the samples are subjected tometal spraying treatment and the image is observed.

X-ray fluorescence spectroscopy (XRF) composition analysis: a GermanBruker S8 Tiger X-ray fluorescence spectrometer is used; 1.2 g of TS-1sample is uniformly mixed with 4 g of boric acid to prepare tablets.

Framework vibration characterization of FT-IR spectrum TS-1 zeolite:characterization is carried out on the IS10 infrared spectrometer ofNicolet company; KBr is used for tabletting; the range of the scanningwave number is 4000-400 cm⁻¹; and the scanning frequency is 64.

X-ray powder diffraction (XRD) crystal structure analysis: D/max·2400X-ray powder diffractometer from Japanese Rigaku company is used formeasurement; CuKα radiation is adopted; the voltage is 40 kV; thecurrent is 100 mA; the range of the scanning diffraction angle is2θ=4-40°; the scanning speed is 2°/min; and the scanning stride is0.08°. The relative crystallinity is obtained according to the ratio ofthe sum of the peak intensities of five MFI structural characteristicpeaks at 2θ=7.8°, 8.8°, 23.2°, 23.8° and 24.3° in the XRD spectrogramand the sum of the intensities of five diffraction peaks (selected) ofthe reference sample.

Embodiment 1. The present embodiment is used to illustrate that thelarge-crystal micron-sized TS-1 zeolite modified by the degreecontrolled hydrothermal treatment method of the alkali metal hydroxidesolution provided by the present invention exhibits high activity andselectivity and utilization rate of hydrogen peroxide for the gas phaseepoxidation of propylene and hydrogen peroxide.

At first step: synthesizing and preparing the large-crystal micron-sizedTS-1 zeolite matrix according to the method introduced in thepublication literature Appl. Catal. A, 185, (1999) 11-18.

The specific feed amount and synthesis steps are as follows:

220 ml of deionized water is added to 225 g of silica sol (26% wt);after stirring for 10 minutes, 18.4 g of tetrapropylammonium bromide isadded to the diluted silica sol solution; after continuing stirring for20 minutes, a silicon solution is obtained; tetrabutyl titanate andacetylacetone are mixed at a mass ratio of 1:0.8, and stirred for 15minutes to prepare a titanium solution; 19.7 ml of the titanium solutionis added to the silicon solution; after stirring for 30 minutes, 57 mlof n-butylamine is added and continuously stirred for 15 minutes toobtain uniform gel; then the obtained gel is added to a 2 l stainlesssteel autoclave and the hydrothermal synthesis is carried out at 170° C.for 96 hours under agitation. After the crystallization time is reached,the hydrothermal crystallization autoclave is naturally cooled to roomtemperature at first, then the autoclave is opened, and the mothersolution is separated by Buchner funnel suction filtration to obtain azeolite filter cake. The filter cake is washed with deionized water forseveral times until the pH value of the washing solution is close to7.0. Then, the filter cake is put into an electric oven and driedovernight at 110° C. The dried solid is then transferred into a mufflefurnace for temperature-programmed calcination to remove the templatingagent. The temperature-programmed calcination starts at roomtemperature, and the temperature is raised to 300° C. at a temperaturerise rate of 10° C./min, and then the temperature is raised from 300° C.to 500° C. at a temperature rise rate of 1° C./min and kept constantuntil the sample is completely white, so as to obtain the large-crystalmicron-sized TS-1 zeolite matrix.

In order to use a reference sample to calculate the relativecrystallinity of the large-crystal micron-sized TS-1 zeolite matrix,embodiment 1 in the Chinese invention patent (application number)201110295555.x is used to prepare the reference sample. Specifically:220 ml of deionized water is added to 225 g of silica sol (20% wt);after stirring for 10 minutes, 18.4 g of tetrapropylammonium bromide and5.1 g of seed crystals are added to the diluted silica sol solution;after continuing stirring for 20 minutes, a silicon solution isobtained; tetrabutyl titanate and acetylacetone are mixed at a massratio of 1:0.8, and stirred for 15 minutes to prepare a titaniumsolution; 19.7 ml of the titanium solution is added to the siliconsolution; after stirring for 30 minutes, 57 ml of n-butylamine is addedand continuously stirred for 15 minutes to obtain uniform gel; then 6.0g of Na₂SO₄ is added and stirred for minutes; and then the obtained gelis added to a 2 l stainless steel autoclave and crystallized at 170° C.for 24 hours under agitation. The post-treatment method of the referencesample is conducted by referring to the processing method of thelarge-crystal micron-sized TS-1 zeolite matrix.

SEM, XRF, FT-IR and XRD are used to characterize the large-crystalmicron-size TS-1 zeolite matrix. Results show that the crystal size is1×2×6 μm, the total Si/Ti molar ratio is about 39.8, and thesodium-titanium molar ratio is 0.003. The index valueI_(960cm−1)/I_(550cm−1) of the framework titanium content is about 0.51and the relative crystallinity is about 100%. The measurement resultsshow that the synthesized large-grained micron TS-1 zeolite matrix meetsthe requirements of the present invention.

At second step: preparing 0.1 mol/L of sodium hydroxide modifiedsolution.

The solution is prepared with analytically pure sodium hydroxide solidparticles (96%) and deionized water. Firstly, 4.17 g of solid sodiumhydroxide particles is accurately weighed. Then, a 1 l volumetric flaskis used to prepare a 0.1 mol/L sodium hydroxide solution (cooled to roomtemperature). For the sake of caution, a standard reagent potassiumhydrogen phthalate and a phenolphthale indicator are used to calibratethe prepared sodium hydroxide solution in accordance with conventionaloperation. A qualified solution has a relative deviation of theconcentration value of less than 5%. Otherwise, the modified solution isprepared again.

At third step: using 0.1 mol/L sodium hydroxide solution to conduct thedegree controlled hydrothermal treatment on the large-crystalmicron-sized TS-1 zeolite matrix.

Specifically, 70 ml of the calibrated 0.1 mol/L sodium hydroxidesolution is accurately measured with a measuring cylinder and added to aplastic cup with magnetic stirrer. Then, 7 g of the large-crystalmicron-sized TS-1 zeolite matrix that is calcined in the first step isweighed, and slowly added into the sodium hydroxide solution underagitation. After the large-crystal micron-sized TS-1 zeolite matrix iscompletely added to the solution, the stirring speed is appropriatelyincreased to make the slurry to a uniform state. The stirring iscontinued for 2 hours at room temperature, and then stopped; and theslurry is transferred into a 100 ml hydrothermal autoclave and sealed.The hydrothermal autoclave is heated in an oven of 170° C. for 18 hoursat constant temperature.

At fourth step: conducting post-treatment of sodium ion modified TS-1zeolite.

After the hydrothermal treatment is ended, the hydrothermal autoclave istaken out of the electric oven and quickly cooled to room temperaturewith tap water. Then the hydrothermal autoclave is carefully opened, andthe mother solution is removed by Buchner funnel suction filtration toobtain a zeolite filter cake. The filter cake is washed with 0.01 mol/Lof sodium hydroxide solution until no precipitate appears after thefiltrate is neutralized with acid. Then, the filter cake is put into theelectric oven and dried overnight at 110° C. to ensure that the drybasis content of the solid powder (solid content measured aftercalcining at 500° C. for 3 hours) is not less than 90%. Finally, thedried solid powder is calcined at a constant temperature of 540° C. for6 hours to obtain the modified zeolite product of embodiment 1.

The sodium ion modified TS-1 zeolite prepared in the present embodimentis tested and evaluated below:

Firstly, infrared spectroscopy is used to characterize the absorptionpeak position of the framework titanium site of the modified TS-1zeolite catalyst.

An appropriate amount of the modified product in the fourth step is putinto a small beaker; an appropriate amount of spectral purity KBr is putinto another small beaker; and the two small beakers are simultaneouslyput into the oven at 110° C. for pre-drying for 4 hours. Then KBr andthe modified TS-1 zeolite product are mixed and ground at a ratio of200:1, and pressed into a wafer under a pressure of 6 MPa; the wafer isput into an infrared sample cell for testing to obtain an infraredspectrum; and finally, the second derivative spectrum in the infraredsoftware is used to accurately locate the infrared characteristicabsorption peak position of the framework titanium active site modifiedby the alkali metal ions, which is at 969 cm⁻¹ for the modified zeoliteproduct of embodiment 1.

In addition, the X-ray fluorescence spectroscopy (XRF) method is used toobtain the silicon-titanium molar ratio and sodium-titanium molar ratioof the modified zeolite product of embodiment 1, which are 37.6 and0.86, respectively.

The characterization results of the infrared spectroscopy and the X-rayfluorescence spectroscopy show that the hydrothermal treatment of thelarge-crystal micron-sized TS-1 matrix with 0.1 mol/L sodium hydroxidesolution produces a controllable silicon dissolution effect, so that thesilicon-titanium molar ratio of the modified catalyst is slightly lowerthan that of the matrix. At the same time, a large amount of sodium ionsexist in the modified catalyst, which makes that the infraredcharacteristic absorption peak of the framework titanium active siteshift from 960 cm⁻¹ (matrix, FIG. 1A) to 969 cm⁻¹ (FIG. 1B). Namely, inthe process of degree controlled hydrothermal treatment modification forthe large-crystal micron-sized TS-1 matrix with 0.1 mol/L sodiumhydroxide solution, the sodium ions replace the hydrogen protons on thesilicon hydroxyl near the framework titanium in the form of countercations, and therefore change the local environment of the nearbyframework titanium site.

Then, a small fixed bed reactor is used to evaluate the gas phaseepoxidation performance of the modified TS-1 zeolite catalyst.

The integrated reactor reported in Chin. J. Catal., 2010, 31: 1195-119is used for a gas phase epoxidation experiment. The upper segment of thereactor is a self-cooling dielectric barrier discharge reactor forin-situ synthesis of gaseous hydrogen peroxide from hydrogen and oxygenplasma. The lower segment of the integrated reactor is a conventionalfixed bed reactor which contains titanium silicalite zeolite particles(20-40 meshes) for the gas phase epoxidation of propylene and hydrogenperoxide. Specific operation steps are as follows: (1) the yield of thehydrogen peroxide is calibrated with the upper segment of plasmareactor: at this moment, the lower segment of reactor should be removed.Firstly, the self-cooling circulating water of the upper segment ofreactor is opened. Then, a hydrogen cylinder and the mass flowcontroller are started to control the hydrogen flow to be 170 ml/min;and next, the oxygen cylinder and the mass flow controller are startedto slowly increase the oxygen flow to be 8 ml/min. During the dischargereaction of the upper segment of reactor, the flows of hydrogen andoxygen should be accurately controlled and hydrogen and oxygen should bemixed uniformly before entering the upper segment of reactor. Then,dielectric barrier discharge is performed according to the dischargemethods introduced in Chinese invention patents (application numbers)200310105210.9, 200310105211.3 and 200310105212.8, so that thehydrogen-oxygen mixture entering the self-cooling dielectric barrierdischarge reactor at the upper segment of the integrated reactorconducts a plasma reaction to produce gaseous hydrogen peroxide. Throughcalibration by conventional iodometry, the yield of the hydrogenperoxide is about 0.35 g/h. (2) The two segments of reactors areintegrated for the gas phase epoxidation of propylene and hydrogenperoxide. After the calibration step, firstly the discharge is stopped,then the oxygen is stopped, and the hydrogen is stopped after 10minutes. 0.5 g of modified large-crystal micron-sized TS-1 zeolitecatalyst (tabletted, crushed, and sieved to obtain 20-40 meshes inadvance according to conventional methods) is loaded into the lowersegment of fixed bed epoxidation reactor, and then the lower segment ofreactor and the upper segment of reactor are connected together. Thelower segment of reactor is inserted into an electric heating furnace.Next, the self-cooling circulating water of the upper segment of reactoris opened. Then, a hydrogen cylinder and the mass flow controller arestarted to control the hydrogen flow to be 170 ml/min; and next, theoxygen cylinder and the mass flow controller are started to slowlyincrease the oxygen flow to be 8 ml/min. The flows of hydrogen andoxygen are accurately controlled and hydrogen and oxygen shall be mixeduniformly before entering the upper segment of reactor. Then, thepropylene feed of the lower segment of reactor is started, and thepropylene flow is controlled as 18 ml/min by the mass flow controller.After the three gas flows are stable and the cooling water flow of theupper segment of reactor is also stable, a plasma power supply of theupper segment of reactor is turned on for dielectric barrier discharge.In this way, the hydrogen peroxide gas synthesized by the discharge ofthe upper segment is carried by excess hydrogen to enter the epoxidationreactor in the lower segment from a gas hole between the two segments ofreactors, and is fully mixed with the propylene gas which enters thesegment of reactor from a side line to jointly enter the TS-1 catalystbed for conducting the epoxidation reaction. The actual molar ratio ofthe propylene and the hydrogen peroxide is calculated to be about 5:1.The reaction temperature of the lower segment of reactor is controlledas 130° C. through the electric heating furnace. After the discharge isconducted for 30 minutes, through an online gas chromatography (analysisby DB-Wax chromatographic column (30 m×0.32 mm, PEG20M) (temperatureprogramming to 50° C. for 5 minutes, at 10° C. to 180° C. per minute for2 minutes, at 20° C. to 200° C. per minute for 5 minutes), the reactionproduct is analyzed, from the analysis data propylene conversion rate iscalculated as 15.5%, the PO selectivity is calculated as 97.0%, and theutilization rate of the hydrogen peroxide is calculated as 77.5%.

Reference embodiment 1. The reference embodiment 1 is used to illustratethat the unmodified large-crystal micron-sized TS-1 zeolite has pooractivity and selectivity for the gas phase epoxidation of propylene andhydrogen, and the utilization rate of hydrogen peroxide is low.

The embodiment 1 is repeated, but the large-crystal micron-sized TS-1zeolite synthesized in the first step is directly used for theevaluation of the gas phase epoxidation without the subsequenthydrothermal modification using the sodium hydroxide solution. Then, thepropylene conversion rate is 4.5%, the PO selectivity is 56.2%, and H₂O₂utilization rate is 22.5%.

Reference embodiment 2. The reference embodiment 2 is used to illustratethat if the large-crystal micron-sized TS-1 is treated according to thesodium exchange method provided in J. Catal., 1995, 151, 77-86, theobtained catalyst has no improvement effect on the gas phase epoxidationof propylene and hydrogen peroxide.

The embodiment 1 is repeated, but the large-crystal micron-sized TS-1zeolite synthesized in the first step is not modified by thehydrothermal modification method of the sodium hydroxide solutionprovided by the present invention, but is modified in accordance withthe sodium exchange method provided by J. Catal., 1995, 151, 77-86. Thespecific method is as follows: 1 mol/L NaOH solution is prepared, andthen 1 g of zeolite matrix is added to 100 mL of 1 mol/L NaOH solution,and stirred at 25° C. for 24 hours. Then the solution is subjected tosuction filtration, dried at 110° C. for 12 hours, and calcined at 540°C. for 6 hours.

Then, the silicon-titanium molar ratio of the sodium exchange catalystmeasured by XFR is reduced to 30, and the sodium-titanium molar ratio is1.40. The infrared characteristic absorption peak of the frameworktitanium measured by the infrared spectroscopy appears at 985 cm⁻¹ (FIG.1C), which is consistent with the literature report. Through comparisonwith the analysis results of the matrix, it can be found that thesilicon-titanium molar ratio of the modified product is reducedsignificantly, indicating that the modification of the sodium exchangemethod reported in the literature for the TS-1 matrix is not acontrollable modification. Instead, it is a method of excessivedissolution of silicon. Although the sodium exchange catalyst contains alarge amount of sodium ions which are combined with the silicon hydroxylin the form of counter cations, which causes the shift of thecharacteristic absorption peak position of the framework titanium from960 cm⁻¹ (matrix) to the high wave number direction, the peak at 985cm⁻¹ is 16 wave numbers higher than the modified catalyst ofembodiment 1. It can be concluded that a substantial difference existsbetween the catalyst obtained by the method of the present invention inembodiment 1 and the catalyst obtained by the sodium exchange method inthe reference embodiment 2.

The evaluation results of the gas phase epoxidation show that thecatalyst prepared by the sodium exchange method reported in theliterature in the reference embodiment 2 has a propylene conversion rateof only 2.3%, a PO selectivity of 81.3%, and a H₂O₂ utilization rate ofonly 10.5%. In other words, the performance of the catalyst modified bythe sodium exchange method in the gas phase epoxidation of propylene andhydrogen peroxide (except for selectivity) is not better than that ofthe matrix. In fact, the catalyst can be considered as basically havingno epoxidation activity. However, the catalyst has high activity for theself-decomposition reaction of the hydrogen peroxide, so that theutilization rate of hydrogen peroxide is only 10.5%.

Reference embodiment 3. The reference embodiment 3 is used to illustratefrom the opposite side that when the large-crystal micron-sized TS-1zeolite is modified according to the degree controlled hydrothermaltreatment method of the alkali metal hydroxide solution provided by thepresent invention, it is important that sodium ions are retained in themodified catalyst.

The embodiment 1 is repeated, but after the operation of the fourth stepis completed, the modified titanium silicalite zeolite is subjected toconventional ammonium exchange treatment twice with 0.4 M ammoniumnitrate at room temperature, each for 2 hours. The engineers familiarwith the field can complete the ammonium exchange work according to themethod described for preparing hydrogen type catalysts through ammoniumexchange of silica-alumina zeolite reported by any publicationliterature. After the ammonium exchange, the solution is removed byBuchner funnel suction filtration to obtain a zeolite filter cake. Then,the filter cake is put into the electric oven and dried overnight at110° C. to ensure that the dry basis content of the solid powder (solidcontent measured after calcining at 500° C. for 3 hours) is not lessthan 90%. Finally, the dried solid powder is calcined at a constanttemperature of 540° C. for 6 hours to obtain an ammonium exchangedzeolite product. Then, the ammonium exchanged zeolite product is used ascatalyst for the gas phase epoxidation. The sodium-titanium ratiosmeasured by XRF for samples of one-time and two-time ammonium exchangeare 0.25 and 0.18 respectively. The infrared spectroscopycharacterization shows that the vibration characteristic absorptionpeaks of the framework titanium of the ammonium exchanged zeoliteproduct are located near 962 cm⁻¹; the conversion rates of the propyleneare 7.6% and 5.7%, respectively; PO selectivities are 83.6% and 34.8%,respectively; and the utilization rates of hydrogen peroxide are 34.6%and 25.9%, respectively.

The reference embodiment 3 illustrates that after the sodium ionmodified TS-1 zeolite obtained in embodiment 1 is subjected to theconventional ammonium exchange, the sodium content (sodium-titaniumratio) is reduced to about 0.2, and at this moment, the infraredvibration characteristic absorption peak of the framework titanium alsomoves from 969 cm⁻¹ (the high sodium state of embodiment 1) back to near960 cm⁻¹. It can be seen from comparison of the reference embodiment 3and embodiment 1 that the decrease of the sodium content in the modifiedzeolite also leads to the significant decrease of the conversion rate ofthe gas phase epoxidation and the utilization rate of hydrogen peroxide.The more the sodium ion content decreases, the more the gas phaseepoxidation performance of the catalyst decreases. This fullydemonstrates that the presence of sufficient sodium ions in the modifiedTS-1 zeolite is the key to obtain a good modification effect in thepresent invention. It can also be seen from the comparison of theselectivity of the propylene oxide that the degree controlled inorganicbase hydrothermal treatment method provided by the present invention mayproduce some acidic sites in the catalyst due to the effect of silicondissolution. The presence of the sodium ions neutralizes the acid sitesat the same time, so that the modified zeolite of embodiment 1 reaches ahigh selectivity of 97%. However, in this reference embodiment, becausemost of the sodium ions are removed through the ammonium exchange,therefore the acid sites produced by the modification is released,thereby causing very low PO selectivity of the ammonium exchange zeolitevia acid catalyzed hydrolysis of propylene oxide.

Reference embodiment 4. The reference embodiment 4 is used to furtherillustrate that when the large-crystal micron-sized TS-1 zeolite ismodified according to the degree controlled hydrothermal treatmentmethod provided by the present invention, it is important that enoughsodium ions are retained in the modified zeolite.

The reference embodiment 3 is repeated, but after the ammonium exchangedcatalyst is obtained, the ammonium exchange catalyst is subjected toreverse exchange treatment of the sodium nitrate solution at roomtemperature for 3 hours. The reverse exchange of the sodium nitratesolution is a conventional ion exchange treatment, and the practice isroughly the same as the ammonium exchange in the reference embodiment 3,except that the ammonium salt solution is changed to a sodium nitratesolution. The engineers familiar with the field can complete the workaccording to the zeolite ion exchange method recorded in any publicationliterature. After the ion exchange of the sodium nitrate solution iscompleted, post treatments including the separation, drying andcalcining are repeated. The obtained sodium nitrate re-exchangedcatalyst is subjected to characterizations and the gas phaseepoxidation.

When the concentrations of the sodium nitrate solution used arerespectively 0.1 M and 0.3 M, the sodium-titanium ratios of the sodiumnitrate exchanged zeolite product measured by XRF are 0.39 and 0.72,respectively; the conversion rates of propylene are 7.6% and 13.3%,respectively; the PO selectivities are 78.6% and 95.2%, respectively;and the utilization rates of hydrogen peroxide are 34.6% and 60.5%,respectively.

The above results further indicate that when the large-crystalmicron-sized TS-1 zeolite is modified according to the degree controlledhydrothermal treatment method of the alkali metal hydroxide solutionprovided by the present invention, it is important that alkali metalions are retained in the modified zeolite. Meanwhile, the referenceembodiment 4 can also illustrate that for the ammonium exchange alkalimetal ion modified zeolite, the lost alkali metal ions can be recoveredto a certain extent through the alkali metal ion reverse exchange,thereby recovering the catalytic performance of the gas phaseepoxidation of the alkali metal ion modified zeolite to a certainextent.

Embodiment 2. The present embodiment is used to illustrate that bychanging a modification time parameter, the degree of hydrothermalmodification of the alkali metal hydroxide solution can be adjusted, andthe catalytic performance of the modified zeolite for the gas phaseepoxidation of propylene and hydrogen peroxide is changed accordingly.

The embodiment 1 is repeated, but in the operation of the third step,the duration of hydrothermal treatment modification is changed to 2, 5,9, 12 and 24 hours, respectively. Then, the relative crystallinity data(FIG. 2 ) of the obtained samples are 82.8%, 82.6%, 86.5%, 84.0% and78.4% in sequence; the silicon-titanium molar ratio data are 37.9, 37.8,37.9, 37.6 and 37.6 in sequence; the sodium-titanium molar ratio dataare 0.91, 0.87, 0.87, 0.85 and 0.75 in sequence; and the positions ofthe infrared characteristic absorption peaks (FIG. 3 ) of the frameworktitanium active sites are at 964, 966, 966, 966 and 970 cm⁻¹ insequence. The results of the gas phase epoxidation of propylene andhydrogen peroxide over the above catalysts are as follows: theconversion rates of propylene are 5.6%, 6.5%, 8.9%, 10.2% and 12.0% insequence; the PO selectivities are 88.5%, 88.6%, 94.3%, 94.6% and 96.9%in sequence; and the utilization rates of hydrogen peroxide are 28.0%,32.5%, 44.5%, 51.0% and 60.0% in sequence. As mentioned above, thehydrothermal treatment time adopted in embodiment 1 is 18 hours, and theconversion rate of propylene, the PO selectivity, and the utilizationrate of hydrogen peroxide of the obtained catalyst are 15.5%, 98.0% and77.5% respectively. It can be seen that the hydrothermal treatment timehas a suitable region. Therefore, the present invention provides apreferred range of 10-20 hours, and a more preferred range of 15-20hours.

However, from the comparison with the reaction result of the matrix(reference embodiment 1), it can be seen that the modificationeffectiveness of the modification method provided by the presentinvention for the modification of the TS-1 zeolite matrix can bereflected in a wide time range.

Embodiment 3. The present embodiment is used to illustrate that bychanging a concentration parameter of the alkali metal hydroxidesolution, the degree of hydrothermal modification can also be adjusted,and the catalytic performance of the modified zeolite for the gas phaseepoxidation of propylene and hydrogen peroxide is changed accordingly.

The embodiment 1 is repeated, but in the operation of the second step,the concentrations of the prepared sodium hydroxide solutions arechanged to 0.05, 0.15, 0.20 and 0.25 mol/L, respectively. Then, theinfrared characteristic absorption peak positions of the frameworktitanium active site of the obtained modified zeolites measured by theinfrared spectroscopy method (FIG. 4 ) are at 972, 971, 967 and 965 cm⁻¹in sequence; the results of the gas phase epoxidation of propylene andhydrogen peroxide over the above modified zeolite are as follows: theconversion rates of propylene are 10.8%, 13.2%, 7.5% and 6.8% insequence; the PO selectivities are 95.1%, 97.5%, 98.0% and 97.9% insequence; and the effective utilization rates of hydrogen peroxide are54.0%, 66.0%, 37.5% and 34.0% in sequence. Considering that theconcentration of the alkali metal hydroxide solution adopted inembodiment 1 is 0.1 mol/L, the conversion rate of propylene, the POselectivity, and the effective utilization rate of hydrogen peroxide ofthe modified zeolite are 15.5%, 97.0% and 77.5% respectively. It can beseen that the concentration of the alkali metal hydroxide solution alsohas a suitable region. Therefore, the present invention provides apreferred range of 0.05-0.2 mol/L, and a more preferred range of0.08-0.15 mol/L.

Similarly, the present invention is intended to state that from thecomparison with the reaction result of the matrix (reference embodiment1), it can be seen that the modification effectiveness of themodification method provided by the present invention for themodification of the TS-1 zeolite matrix can be reflected in a wideconcentration range of the alkali metal hydroxide solution.

Embodiment 4. The present embodiment is used to illustrate that bychanging a temperature parameter, the degree of hydrothermalmodification of the alkali metal hydroxide solution can be adjusted, andthe catalytic performance of the modified zeolite for the gas phaseepoxidation of propylene and hydrogen peroxide is changed accordingly.

The embodiment 1 is repeated, but in the operation of the third step,the temperatures of hydrothermal treatment modification are changed to25° C., 80° C., 110° C., 150° C., 190° C. and 210° C., respectively.Then, the results of the gas phase epoxidation of propylene and hydrogenperoxide over the obtained modified zeolites are as follows: theconversion rates of propylene are 4.2%, 6.3%, 9.4%, 13.7%, 12.5% and7.8% in sequence; the PO selectivities are 90.1%, 92.6%, 97.2%, 97.0%,96.6% and 97% in sequence; and the utilization rates of hydrogenperoxide are 21.0%, 31.5%, 47.0%, 68.5%, 62.5% and 39.0% in sequence.Considering that the hydrothermal treatment temperature adopted inembodiment 1 is 170° C., the conversion rate of propylene, the POselectivity, and the utilization rate of hydrogen peroxide of theobtained modified zeolite are 15.5%, 97.0% and 77.5% respectively. Itcan be seen that the hydrothermal treatment temperature also has asuitable region. Therefore, the present invention provides a preferredrange of 100-200° C., and a more preferred range of 150-190° C.

Herein, the present invention is intended to state that from thecomparison with the reaction result of the matrix (reference embodiment1), it can be seen that the modification effectiveness of themodification method provided by the present invention for themodification of the TS-1 zeolite matrix can be reflected in a wide rangeof the hydrothermal treatment temperature.

Embodiment 5. The present embodiment is used to illustrate that byregulating a liquid-solid ratio parameter, the degree of hydrothermalmodification of the alkali metal hydroxide solution can be adjusted, andthe catalytic performance of the modified zeolite for the gas phaseepoxidation of propylene and hydrogen peroxide is changed accordingly.

The embodiment 1 is repeated, but in the operation of the third step,the liquid-solid ratios of hydrothermal treatment modification arechanged to 4, 5, 7 and 15, respectively. The results of the gas phaseepoxidation of propylene and hydrogen peroxide in the modified zeolitesare as follows: the conversion rates of propylene are 9.7%, 12.6%, 13.5%and 10.8% in sequence; the PO selectivities are 95.2%, 95.7%, 97.3% and97.5% in sequence; and the effective utilization rates of hydrogenperoxide are 48.5%, 63.0%, 67.5% and 54.0% in sequence. Similarly,considering that the liquid-solid ratio adopted in embodiment 1 is 10,the conversion rate of propylene, the PO selectivity, and theutilization rate of hydrogen peroxide of the obtained modified zeoliteare 15.5%, 97.0% and 77.5% respectively. Obviously, the liquid-solidratio also has a suitable region. Therefore, the present inventionprovides a preferred range of 5-15, and a more preferred range of 8-12.

Therefore, from the comparison with the reaction result of the matrix(reference embodiment 1), it can be seen that the modificationeffectiveness of the modification method provided by the presentinvention for the modification of the TS-1 matrix can be reflected in awide range of the liquid-solid ratio.

Embodiment 6. The present embodiment is used to illustrate that in thewashing step after the hydrothermal treatment, the use of a suitablelow-concentration alkali metal hydroxide solution as the washingsolution is beneficial to achieve the modification effect.

The embodiment 1 is repeated, but in the post-treatment washing step ofthe fourth step, deionized water, and 0.001, 0.005 and 0.05 mol/L sodiumhydroxide solutions are used to wash the filter cake, respectively. Whenno precipitation appears after the filtrate is neutralized, thesodium-titanium molar ratio data of the obtained modified zeolite are0.48, 0.80, 0.85 and 0.88 in sequence. The results of the gas phaseepoxidation of propylene and hydrogen peroxide in the above catalystsare as follows: the conversion rates of propylene are 10.1%, 14.3%,15.6% and 15.2% in sequence; the PO selectivities are 86.7%, 96.5%,96.4% and 96.9% in sequence; and the utilization rates of hydrogenperoxide are 50.5%, 71.5%, 78.0% and 76.0% in sequence.

Embodiment 7. The present embodiment is used to illustrate that when thelarge-crystal micron-sized TS-1 is modified according to the degreecontrolled hydrothermal treatment method of the alkali metal hydroxidesolution provided by the present invention, potassium hydroxide is alsoeffective.

The embodiment 1 is repeated, but in the second step of preparing thehydrothermal modification solution, the potassium hydroxide is used toreplace the sodium hydroxide. Then, after the obtained catalyst isanalyzed by XRF, the silicon-titanium molar ratio is 37.4 and apotassium-titanium molar ratio is 0.84. The results of the gas phaseepoxidation of propylene and hydrogen peroxide presented by the samplein a fixed bed reactor are: the conversion rate of propylene is 15.0%,the PO selectivity is 97.2% and the utilization rate of hydrogenperoxide is 75.0%.

Embodiment 8. The present embodiment is used to illustrate that when thelarge-crystal micron-sized TS-1 is modified according to the degreecontrolled hydrothermal treatment method of the alkali metal hydroxidesolution provided by the present invention, lithium hydroxide is alsoeffective.

The embodiment 1 is repeated, but in the second step of preparing thehydrothermal modification solution, the lithium hydroxide is used toreplace the sodium hydroxide. Then, the results of the gas phaseepoxidation reaction of propylene and hydrogen peroxide over theobtained catalyst are: the conversion rate of propylene is 14.5%, the POselectivity is 96.6% and the utilization rate of hydrogen peroxide is72.5%.

Embodiment 9. The present embodiment is used to illustrate that thehydrothermal treatment method provided by the present invention can beapplicable to a small-crystal micron-sized TS-1 zeolite matrix.

The embodiment 1 is repeated, but in the first step of hydrothermalsynthesis of the TS-1 zeolite matrix, the small-crystal TS-1 zeolitematrix that can be used in the present invention is synthesizedaccording to the reference embodiment 1 of the Chinese invention patent(application number) 201310691060.8. The crystal size of the sampleprovided by the scanning electron microscope (SEM) is about 0.5 micron(FIG. 5 ). Then, the results of the gas phase epoxidation of propyleneand hydrogen peroxide over the modified zeolite are: the conversion rateof propylene is 14.7%, the PO selectivity is 96.6% and the utilizationrate of hydrogen peroxide is 73.5%.

Reference embodiment 5. The reference embodiment 5 is used to illustratethat the modified TS-1 zeolite obtained according to the method of thepresent invention has improvement effects on the gas phase epoxidationof propylene and hydrogen peroxide, but has no obvious improvementeffect on the liquid phase epoxidation of propylene and hydrogenperoxide.

The liquid phase epoxidation can be conducted according to the methodintroduced by any publication literature. Specifically, in the referenceembodiment 5, the liquid phase epoxidation is conducted in a 450 mlstainless steel reactor under water bath temperature control andmagnetic stirring. Experimental conditions are as follows: the reactiontemperature is 40° C., the propylene pressure is 0.6 MPa, and thereaction time is 1 h. The ingredients are as follows: 0.2 g of catalyst,30 ml of methanol and 2 ml of H₂O₂ (30%). Before the experiment, thereactor is pressurized with propylene gas, and then gas is vented. Thereplacement is repeated for 5-6 times in this way for the purpose ofreplacing the air in the reactor. The concentration of H₂O₂ in theproduct solution is measured by iodometry, and the reaction product isanalyzed by chromatography.

In the reference embodiment 5, the zeolite samples of embodiment 1 andembodiment 2 are used respectively for the liquid phase epoxidationreaction. See Table 1 for the results. It can be seen from Table 1 thatif the modified zeolite prepared by the method of the present inventionis used in the liquid phase epoxidation reaction, the conversion rate ofhydrogen peroxide is reduced, and the utilization rate of hydrogenperoxide is also reduced. The selectivity improvement effect of themodified zeolite in the liquid phase epoxidation is actually the resultof neutralizing a small amount of weakly acidic sites on the surface ofthe zeolite by the sodium ions. These are consistent with the resultsobtained on the sodium exchange TS-1 zeolite by J. Catal., 1995, 151,77-86. The important information to be emphasized in the referenceembodiment 5 is: the alkali metal ion modified framework titanium activesite obtained by the method of the present invention, i.e., the degreecontrolled hydrothermal modification method of the alkali metalhydroxide solution, is also not conducive to the liquid phase oxidation.The presence of the sodium ions on the silicon hydroxyl near theframework titanium hinders the liquid phase oxidation reaction (reducesthe conversion rate), but is relatively conducive to theself-decomposition reaction of hydrogen peroxide (reduces theutilization rate). The experimental results confirm that the frameworktitanium active site modified by the alkali metal ions is conducive tothe gas phase epoxidation, which is an important discovery.

TABLE 1 Liquid Phase Epoxidation Data of Propylene and Hydrogen PeroxideObtained with Catalysts of Embodiments 1 and 2 in Reference Embodiment 5Liquid Phase Epoxidation (HPPO) Data of Propylene and Hydrogen PeroxideS(PO)/ U(H₂O₂)/ Sample X(H₂O₂)/% % % umTS-1 36.1 90.7 87.3 Hydrothermal2 h 18.1 99.2 48.0 Hydrothermal 5 h 6.9 97.6 67.8 Hydrothermal 9 h 8.498.7 71.6 Hydrothermal 12 h 6.2 98.3 73.7 Hydrothermal 18 h 13.8 98.946.3 Hydrothermal 24 h 13.0 98.7 48.9

Reference embodiment 6. The reference embodiment 6 is used to illustratethat the hydrothermal treatment method provided by the present inventionis not applicable to the nano TS-1 zeolite matrix synthesized by theclassical method.

The embodiment 1 is repeated, but in the first step of hydrothermalsynthesis of the TS-1 zeolite matrix, the TS-1 matrix is synthesizedaccording to the formula of the classical method introduced by theChinese invention patent (application number) 200910131993.5. Thesilicon-titanium molar ratio, the framework titanium index data, and therelative crystallinity index of the matrix meet the requirements of thepresent invention, but the crystal size is 200-300 nanometers(aggregates). Thus, the matrix belongs to nanosized TS-1 and is aninapplicable matrix as mentioned above in the present invention.However, in order to illustrate it with the reaction results, the nanoTS-1 is modified according to the procedure introduced in theembodiment. Then, the results of the gas phase epoxidation of propyleneand hydrogen peroxide are obtained as follows: for the nano TS-1 zeolitematrix, the conversion rate of propylene is 7.3%, the PO selectivity is76.7% and the utilization rate of hydrogen peroxide is 36.5%; In thecase of the modified nano TS-1 zeolite, however, the conversion rate ofpropylene is 0.42%, the PO selectivity is 86.2% and the utilization rateof hydrogen peroxide is 2.1%.

1. A preparation method of an alkali metal ion modified titaniumsilicalite zeolite for the gas phase epoxidation of propylene andhydrogen peroxide, wherein in the alkali metal ion modified titaniumsilicalite zeolite, alkali metal ions are reserved on the siliconhydroxyl of the modified TS-1 zeolite; an infrared characteristicabsorption band of framework titanium active site modified by the alkalimetal ions is in a range above 960 cm⁻¹ and below 980 cm⁻¹; TS-1 zeolitematrix of the alkali metal ion modified titanium silicalite zeolitemeets the following requirements: the crystal size is ≥0.3 micron; asilicon-titanium molar ratio is ≤200; an index value of the frameworktitanium content is ≥0.40; and relative crystallinity is ≥85%; whereinspecific steps of conducting a degree controlled hydrothermal treatmenton the TS-1 zeolite matrix by using an alkali metal hydroxidemodification solution in the preparation method are as follows: at firststep: preparing an alkali metal hydroxide modification solution; theconcentration of the alkali metal hydroxide modification solution is0.05-0.2 mol/L; at second step: conducting the degree controlledhydrothermal treatment on the TS-1 zeolite matrix by using the alkalimetal hydroxide modification solution; the ratio of volume of the alkalimetal hydroxide modification solution to weight of the TS-1 zeolitematrix is in the range of 5-15 ml/g; hydrothermal modificationtemperature is 100° C.-200° C.; hydrothermal modification time is 10-20hours; at third step: conducting post-treatment on the hydrothermallymodified TS-1 zeolite; the post-treatment comprises solid-liquidseparation, washing, drying and calcining; in the washing process, themodified TS-1 zeolite wet material obtained by solid-liquid separationis washed by using a low concentration alkali metal hydroxide solution,and the degree of washing is satisfactory when no precipitate appearsafter the washing solution is neutralized with acid; the concentrationof the alkali metal hydroxide solution for washing is 0.001-0.05 mol/L.2. The preparation method of the alkali metal ion modified titaniumsilicalite zeolite for the gas phase epoxidation of propylene andhydrogen peroxide according to claim 1, wherein in the first step, theconcentration of the alkali metal hydroxide modification solution is0.08-0.15 mol/L; and the alkali metal hydroxide is lithium hydroxide,sodium hydroxide or potassium hydroxide.
 3. The preparation method ofthe alkali metal ion modified titanium silicalite zeolite for the gasphase epoxidation of propylene and hydrogen peroxide according to claim1, wherein in the second step, the ratio of volume of the alkali metalhydroxide modification solution to weight of the TS-1 zeolite matrix isin the range of 8-12 ml/g; hydrothermal modification temperature is 150°C.-190° C.; and hydrothermal modification time is 15-20 hours.
 4. Thepreparation method of the alkali metal ion modified titanium silicalitezeolite for the gas phase epoxidation of propylene and hydrogen peroxideaccording to claim 2, wherein in the second step, the ratio of volume ofthe alkali metal hydroxide modification solution to weight of the TS-1zeolite matrix is in the range of 8-12 ml/g zeolite; hydrothermalmodification temperature is 150° C.-190° C.; and hydrothermalmodification time is 15-20 hours.
 5. The preparation method of thealkali metal ion modified titanium silicalite zeolite for the gas phaseepoxidation of propylene and hydrogen peroxide according to claim 1,wherein in the third step, the concentration of the alkali metalhydroxide solution used for washing is 0.005-0.04 mol/L. the alkalimetal hydroxide is lithium hydroxide, sodium hydroxide or potassiumhydroxide; the drying temperature is 80-120° C., and the drying time isdecided based on the dry basis content of the sample not less than 90%;the final calcining temperature is 400-550° C., and the constanttemperature time at the final calcining temperature is more than 3hours.
 6. The preparation method of the alkali metal ion modifiedtitanium silicalite zeolite for the gas phase epoxidation of propyleneand hydrogen peroxide according to claim 2, wherein in the third step,the concentration of the alkali metal hydroxide solution used forwashing is 0.005-0.04 mol/L; the alkali metal hydroxide is lithiumhydroxide, sodium hydroxide or potassium hydroxide; the dryingtemperature is 80-120° C., and the drying time is decided based on thedry basis content of the sample not less than 90%; the final calciningtemperature is 400-550° C., and the constant temperature time at thefinal calcining temperature is more than 3 hours.
 7. The preparationmethod of the alkali metal ion modified titanium silicalite zeolite forthe gas phase epoxidation of propylene and hydrogen peroxide accordingto claim 5, wherein in the fourth step, the concentration of the alkalimetal hydroxide solution used for washing is 0.005-0.03 mol/L.
 8. Thepreparation method of the alkali metal ion modified titanium silicalitezeolite for the gas phase epoxidation of propylene and hydrogen peroxideaccording to claim 6, wherein in the fourth step, the concentration ofthe alkali metal hydroxide solution used for washing is 0.005-0.03mol/L.
 9. The preparation method of the alkali metal ion modifiedtitanium silicalite zeolite for the gas phase epoxidation of propyleneand hydrogen peroxide according to claim 3, wherein in the third step,the concentration of the alkali metal hydroxide solution used forwashing is 0.005-0.04 mol/L; the alkali metal hydroxide is lithiumhydroxide, sodium hydroxide or potassium hydroxide; the dryingtemperature is 80-120° C., and the drying time is decided based on thedry basis content of the sample not less than 90%; the final calciningtemperature is 400-550° C., and the constant temperature time at thefinal calcining temperature is more than 3 hours.
 10. The preparationmethod of the alkali metal ion modified titanium silicalite zeolite forthe gas phase epoxidation of propylene and hydrogen peroxide accordingto claim 9, wherein in the fourth step, the concentration of the alkalimetal hydroxide solution used for washing is 0.005-0.03 mol/L.
 11. Thepreparation method of the alkali metal ion modified titanium silicalitezeolite for the gas phase epoxidation of propylene and hydrogen peroxideaccording to claim 1, wherein the crystal size of the TS-1 zeolitematrix is ≥0.5 micron; the silicon-titanium molar ratio is ≤100; theindex value of the framework titanium content is ≥0.45; and the relativecrystallinity is ≥90%.